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Computed value for this example circuit, not a service limit.
Tap a part of the drawing, or move to it with the Tab key and press Enter, to read what it is.
Tap a test point ring, or move to it with the Tab key and press Enter, to place the selected lead. The names of the points the leads are on show under the reading, and the meter reads as soon as both leads are on.
Pick a scenario, read the symptom, set the controls, take readings, then choose the cause.
A tap on a test point places the selected lead, and the meter reads as soon as both leads are on. Across a part means one lead on each side of it: across the switch is fuse 1 output to switch output.
Keep practising: electrical questions for your trade
This is a study simulation for trade exams, not a test or repair procedure. Readings are computed values for this example circuit, not service limits.
Measure an example 12 V and 24 V lamp circuit with a virtual meter, then find hidden opens, shorts and high-resistance connections from the readings.
This free simulator puts a virtual meter on an example 12 V lamp circuit, and on a 24 V view of the same drawing with two 12 V batteries in series. It is for trade apprentices studying DC circuits, Ohm’s law and voltage-drop testing. In Explore, tap a part of the drawing to read what it does. In Meter, place the red and black leads on two test points to read the voltage, or disconnect the battery and pick Ω to read resistance. In Find the fault, a hidden fault is set: read the symptom, take readings, then choose the cause. Every reading is computed for this example circuit from Ohm’s law and the series and parallel rules, and none of them is a service limit.
In Meter mode each wire is drawn by its voltage above the battery negative post, and the width and dash pattern back up the colour; with the battery disconnected, wires joined to its positive post stay near the supply voltage. Moving arrows show where current flows, from the positive side toward the negative side. A lamp shows six rays when bright, two when dim and none when off; a blown fuse shows a broken link, and a removed fuse an empty dashed holder. Test points are the small rings, and the markers with a plus and a minus sign show where the red and black leads sit.
Near the supply voltage. Thick solid red-orange line with a dark outline. Close to the full supply voltage: little has been lost between the battery positive post and this wire.
Part of the supply voltage. Thick orange line with long dashes and a dark outline. Between the supply voltage and 0 V: the cable between the two batteries in the 24 V view, or a part with voltage across it, such as an open switch or a removed fuse.
Near ground. Thin solid blue line. Close to 0 V, the voltage of the battery negative post: the return side of the circuit, or a feed wire that no voltage reaches.
In Find the fault the wires are drawn grey with no current arrows, so only the lamps, the fuses and the readings taken show the evidence.
A voltmeter is connected across the part being measured, in parallel with it. On DC volts, a minus sign on the display means the leads are the other way round. In a series path the same current flows through every part, and the voltage drops across the parts add up to the source voltage. Each drop is the current times that part’s resistance, so the part with the most resistance takes the most voltage. In this example circuit, with the lamps on, the meter across the battery posts reads 12.30 V and lamp 1 takes 11.93 V; the wires, fuse, switch, connector and ground connections share what is left.
When a series path is open, no current flows, so no resistive part drops any voltage and the whole source voltage stands across the open. Close to the full source voltage across a switch means the switch is open, but zero across a switch does not prove that it is closed. In this example circuit, with the switch off, the meter across the switch reads 12.46 V and lamp 1 positive terminal reads 0.00 V to chassis ground. Parts that other lamps still use, such as the battery and its cables, keep a small drop, smaller with fewer lamps on, so in this example circuit the reading across an open is close to the full source voltage. With an open on the feed side of a load, the points before the open read the source voltage to ground and the points past it read 0 V.
A voltage drop exists only while current flows, so the circuit is switched on and loaded before the meter goes across the section under test. The feed side and the ground side are measured separately, and their drops are added to give the loss for the whole circuit. With no current, even a bad connection drops no voltage, so a voltage check with the circuit off reads normal. In this example circuit, a faulty lamp ground connection reads 0.00 V to chassis ground with the switch off and 3.02 V with the lamps on.
This simulator gives no verdict on a single reading. In Find the fault, each reading is shown with the same test on a healthy circuit, so the two can be compared. In this example circuit, with a bad lamp ground connection, the ground connection reads 3.02 V to chassis ground while across the switch reads 0.03 V, which points to the ground side. On a real circuit, readings are compared with the service information for that circuit.
An ohmmeter drives its own current from an internal battery, so it is used only on a circuit with the power off. Before a resistance check, the power is turned off and the part is removed or isolated. In this example circuit, with the battery disconnected and fuse 1 removed, the wire side of the lamp 1 connector reads 0.3 Ω to chassis ground through the two unlit lamps, and OL once both lamps are unplugged. In this example circuit, that OL means the resistance is above what the meter can read: with both lamps unplugged, no path is left from the wire to chassis ground. This simulator shows no resistance reading while the battery is connected. Voltage testing under load is preferred: a resistance check means disconnecting wiring, which can disturb the fault, and by Ohm’s law a small resistance takes more voltage as the current through it grows. In this example circuit, with the battery disconnected and fuse 1 removed, a faulty lamp ground connection reads 0.6 Ω, against 0.2 Ω healthy, yet it takes 3.02 V with the lamps on. In this example circuit the meter leads themselves read 0.2 Ω, close to zero, and that is part of every reading in ohms.
Lamps in parallel each get the same voltage and work on their own, so when one burns out the others keep working. The total current is the sum of the currents in the parallel paths, so a part that several lamps share carries all of their current. In the 24 V view of this example circuit, a bad cable between the batteries reads 7.94 V with every lamp on and 3.38 V with only the circuit 2 lamp on. In the 12 V view of this example circuit, a bad ground connection that lamps 1 and 2 share dims both of them, while a bad lamp 1 connector dims lamp 1 alone.
In the 24 V view two 12 V batteries are joined in series by a short cable, so their voltages add, and so do their internal resistances; the lamps there are 24 V lamps of the same power. The positive cable, the cable between the batteries and the negative cable are then three sections, each measured on its own under load. In the 24 V view of this example circuit, with every lamp on, the healthy cable between the batteries reads 0.02 V, and the healthy positive and negative cables 0.03 V each. In the 24 V view, the battery positive post is the positive post of battery 2, where the positive cable starts, and the battery negative post is that of battery 1, where the negative cable starts.
Symbol: Two pairs of long and short plates, the long plate on the positive side, marked with plus and minus signs.
What it does: The battery is the power source of the circuit. In the 24 V view two 12 V batteries are joined in series, so their voltages add.
If it fails: A battery has internal resistance, so its terminal voltage falls as the current rises. As a battery runs down its internal resistance rises, and every lamp it feeds dims.
Symbol: Lines from the battery posts: the positive cable to the fuse block, the negative cable to the chassis, and in the 24 V view a short cable between the two batteries.
What it does: The battery cables carry the current of every circuit in the drawing. The negative cable joins the battery to the chassis, which is the return path.
If it fails: A poor connection at a cable end loses voltage, and the loss grows with the current through it. In the 24 V view of this example circuit, a bad positive or negative cable reads 7.94 V across that cable, and every lamp is dim.
Symbol: A box labelled Fuse block, holding fuse 1 and fuse 2.
What it does: The fuse block takes the battery positive feed and splits it into two fused circuits: fuse 1 feeds the switch and lamps 1 and 2, and fuse 2 feeds the circuit 2 lamp.
If it fails: A poor connection at a fuse block terminal loses voltage, and if the drop is large the lamps it feeds work poorly or not at all.
Symbol: A small box with a link through it. A blown fuse shows a broken link, and a removed fuse an empty dashed holder.
What it does: A fuse is the protection in the circuit, in series with the parts it feeds. It opens by melting when the current is too high, and a real fuse opens faster the larger the overcurrent.
If it fails: A blown fuse is an open, so the lamps it feeds go dark. In this example circuit, after a short to ground opens fuse 1, the meter across the fuse reads 12.46 V with the switch on. On a real circuit, the cause is found before a new fuse goes in, and the new fuse has the rating the service information gives, because a larger one can let the wire overheat.
Symbol: A blade between two terminals, drawn closed or open.
What it does: The switch controls lamps 1 and 2 and is in series with them.
If it fails: Close to the full source voltage across a switch means it is open; across a closed switch a meter may show a very small reading from slight contact resistance, and a larger reading under load means resistance in the contacts. In this example circuit, worn contacts read 3.02 V across the closed switch with the lamps on, against 0.04 V healthy.
Symbol: A line between two parts. In Meter mode its colour, width and dash pattern show its voltage, and moving arrows show current.
What it does: Wires are the conductors that carry current between the parts. A wire’s resistance rises with its length and temperature and falls as its cross-section grows.
If it fails: A broken wire is an open: no current flows through it, and with the circuit switched on, close to the full source voltage appears across the break. A feed wire that touches the chassis past the fuse is a short to ground: the current becomes very large and the fuse opens.
Symbol: Two halves with a pin between them; unplugged, the lamp-side half moves away.
What it does: The connector joins a lamp to its feed wire, and unplugging it isolates the lamp for a resistance check.
If it fails: A connector that has come apart is an open, and a poor one loses voltage under load. In this example circuit, a high-resistance lamp 1 connector leaves lamp 1 positive terminal at 8.82 V to chassis ground, against 12.02 V healthy.
Symbol: A circle with a cross; rays around it show how bright it is.
What it does: The lamp is the load of the circuit. In this example circuit, with the lamps on, lamp 1 takes 11.93 V of the 12.30 V across the battery posts.
If it fails: Correct voltage across a load that does not work means the load itself has failed. In this example circuit, a burned-out lamp 1 reads 12.25 V across its terminals with the switch on.
Symbol: A ring terminal bolted to the chassis, drawn as a nut.
What it does: A ground connection joins lamp ground wires to the chassis, which carries the current back to the battery. Lamps 1 and 2 share one ground connection, and the circuit 2 lamp has its own.
If it fails: A poor ground connection loses voltage like any poor connection, and if the drop is large the lamps on it work poorly or not at all. In this example circuit, a bad lamp ground connection reads 3.02 V to chassis ground with the lamps on, against 0.05 V healthy.
Symbol: A long bar with hatching along the bottom of the drawing.
What it does: In a single-wire system the feed side is wired and the chassis is the return path to the battery; a part can also have its own ground wire to the chassis. Here ground means the chassis return path, which carries the circuit current; it is not the same as the ground wire of building wiring. For a reading to chassis ground, the black lead goes on it.
If it fails: A poor connection to the chassis, at a ground connection or at the battery negative cable, loses voltage under load like any poor connection.
| Fault | Symptom | What to test | What you would see | Cause |
|---|---|---|---|---|
| Lamp 1 burned out | Lamp 1 does not light. Lamp 2 works. | With the switch on, voltage from lamp 1 positive and ground terminals to chassis ground, and across the lamp 1 connector. | In this example circuit, lamp 1 positive terminal reads 12.27 V to chassis ground and its ground terminal 0.03 V. In this example circuit, across lamp 1 reads 12.25 V and across the connector 0.00 V. | Lamp 1 burned out: the voltage reaches the lamp, but the lamp is open inside. |
| Open at the lamp 1 connector | Lamp 1 does not light. Lamp 2 works. | With the switch on, voltage from lamp 1 positive and ground terminals to chassis ground, and across the lamp 1 connector. | In this example circuit, across the lamp 1 connector reads 12.25 V, against 0.01 V healthy, and both lamp 1 terminals read 0.03 V to chassis ground. | Open at the lamp 1 connector: the supply voltage stands across the gap. |
| Open in the lamp 1 ground wire | Lamp 1 does not light. Lamp 2 works. | With the switch on, voltage from lamp 1 positive and ground terminals to chassis ground, and across the lamp 1 connector. | In this example circuit, lamp 1 positive and ground terminals both read 12.27 V to chassis ground, and across the connector reads 0.00 V. | Open in the lamp 1 ground wire: voltage on both sides of the lamp, and no current. |
| Switch does not close | Lamps 1 and 2 do not light. The circuit 2 lamp works. | With the switch on, voltage from fuse 1 output and from the switch output to chassis ground. | In this example circuit, with the switch on, fuse 1 output reads 12.46 V to chassis ground and the switch output 0.00 V, and lamp 2 is dark along with lamp 1. | Switch does not close: no voltage past the switch, and both lamps it feeds are dark. |
| High resistance across the switch contacts | Lamps 1 and 2 are both dim. The circuit 2 lamp is normal. | With the lamps on, voltage across the switch and from the lamp ground connection to chassis ground. | In this example circuit, across the closed switch reads 3.02 V with the lamps on, against 0.04 V healthy, and the lamp ground connection reads 0.04 V to chassis ground. | High resistance across the switch contacts: the feed side takes the volts the lamps should get. |
| High resistance at the lamp ground connection | Lamps 1 and 2 are both dim. The circuit 2 lamp is normal. | With the lamps on, voltage across the switch and from the lamp ground connection to chassis ground. | In this example circuit, the lamp ground connection reads 3.02 V to chassis ground with the lamps on, against 0.05 V healthy, and across the switch reads 0.03 V. In the 24 V view, where the lamps draw less current, this example circuit reads 1.90 V there, against 0.03 V healthy. The circuit 2 lamp has its own ground connection, so it stays normal. | High resistance at the lamp ground connection: the ground side takes the volts the lamps should get. |
| High resistance at the lamp 1 connector | Lamp 1 is dim. Lamp 2 and the circuit 2 lamp are normal. | With the lamps on, voltage from the switch output and from lamp 1 positive terminal to chassis ground. | In this example circuit, lamp 1 positive terminal reads 8.82 V to chassis ground, against 12.02 V healthy, while the switch output reads 12.14 V. Only lamp 1 is dim, and lamp 2 stays bright. | High resistance at the lamp 1 connector: lamp 1 alone loses volts. |
| Fuse rating too low for the circuit | Fuse 1 opens with the switch on and both lamps plugged in. | With the battery disconnected, fuse 1 removed and both lamps unplugged, resistance from the wire side of the lamp 1 connector to chassis ground. | In this example circuit, with the battery disconnected, fuse 1 removed and both lamps unplugged, the wire side of the lamp 1 connector reads OL to chassis ground, the same as a healthy circuit. With either lamp unplugged, a new fuse 1 holds in this example circuit. | Fuse rating too low for the circuit: it opens on the normal current of the lamps. |
| Lamp 1 feed wire shorted to ground | Fuse 1 opens as soon as the switch is turned on. | With the battery disconnected, fuse 1 removed and both lamps unplugged, resistance from the wire side of the lamp 1 connector to chassis ground. | In this example circuit, with the battery disconnected, fuse 1 removed and both lamps unplugged, the wire side of the lamp 1 connector reads 0.2 Ω to chassis ground, against OL healthy. In this example circuit, the short leaves lamps 1 and 2 dark, and a new fuse 1 opens again as soon as the switch is on. | Lamp 1 feed wire shorted to ground: a low-resistance path opens fuse 1. |
| Short to ground on the lamp side of the lamp 1 connector | Fuse 1 opens as soon as the switch is turned on. | With the battery disconnected, fuse 1 removed and both lamps unplugged, resistance from the wire side of the lamp 1 connector to chassis ground. | In this example circuit, with the battery disconnected, fuse 1 removed and both lamps unplugged, the wire side of the lamp 1 connector reads OL to chassis ground, the same as a healthy circuit. With lamp 1 unplugged, a new fuse 1 holds in this example circuit. | Short to ground on the lamp side of the lamp 1 connector: unplugging lamp 1 removes it. |
| Lamp 1 feed wire touching a powered wire | Lamps 1 and 2 stay on with the switch off. | With the switch off and fuse 1 removed, voltage from fuse 1 output and from lamp 1 positive terminal to chassis ground. | In this example circuit, lamp 1 positive terminal still reads 12.00 V to chassis ground with the switch off and fuse 1 removed, against 0.00 V healthy, and the lamps stay on. Fuse 1 output reads 0.00 V in the same state in this example circuit, so the voltage reaches lamp 1 from past the switch. | Lamp 1 feed wire touching a powered wire: voltage from another circuit keeps the lamps on. |
| Switch contacts stuck closed | Lamps 1 and 2 stay on with the switch off. | With the switch off, voltage across the switch, then from lamp 1 positive terminal to chassis ground with fuse 1 removed. | In this example circuit, across the switch reads 0.04 V with the switch off, as across a closed switch, against 12.46 V healthy. Once fuse 1 is removed, lamp 1 positive terminal reads 0.00 V to chassis ground in this example circuit, and the lamps go out. | Switch contacts stuck closed: the path through fuse 1 stays complete. |
| Battery cannot hold its voltage under load | Every lamp is dim, the circuit 2 lamp too. | With the lamps on, voltage from the fuse block input to chassis ground, across the switch, and from the lamp ground connection to chassis ground. | In this example circuit, the fuse block input reads 9.14 V to chassis ground with the lamps on, against 12.18 V healthy. Across the switch reads 0.03 V and the lamp ground connection 0.04 V in this example circuit, close to healthy, and the circuit 2 lamp dims too. | Battery cannot hold its voltage under load: the supply sags for every circuit. |
| High resistance in the battery negative cable | Every lamp is dim, the circuit 2 lamp too. | With the lamps on in the 24 V view, voltage across the cable between the batteries, the battery positive cable, the battery negative cable and the lamp ground connection. | In the 24 V view of this example circuit, red on chassis ground and black on the battery negative post reads 7.94 V, against 0.03 V healthy, while the cable between the batteries reads 0.02 V. | High resistance in the battery negative cable: the return path takes volts from every circuit. |
| High resistance in the battery positive cable | Every lamp is dim, the circuit 2 lamp too. | With the lamps on in the 24 V view, voltage across the cable between the batteries, the battery positive cable, the battery negative cable and the lamp ground connection. | In the 24 V view of this example circuit, from the battery positive post to the fuse block input reads 7.94 V, against 0.03 V healthy, while the cable between the batteries and the negative cable read 0.02 V. | High resistance in the battery positive cable: the feed path takes volts from every circuit. |
| High resistance in the cable between the batteries | Every lamp is dim, the circuit 2 lamp too. | With the lamps on in the 24 V view, voltage across the cable between the batteries, the battery positive cable, the battery negative cable and the lamp ground connection. | In the 24 V view of this example circuit, across the cable between the batteries reads 7.94 V, against 0.02 V healthy, while both battery cables read 0.02 V. | High resistance in the cable between the batteries: a shared path takes volts from every circuit. |
Correct voltage across a load that does not work means the load itself has failed. A burned-out lamp is open inside, so the supply voltage stands across its terminals.
In this example circuit, an open at the connector would read 0.03 V on lamp 1 positive terminal, and an open ground wire 12.27 V on its ground terminal.
With the connector open, no current flows through lamp 1, so lamp 1 and its own wires drop no voltage and nearly the whole supply voltage stands across the gap. Lamp 2 has its own path from the switch, so it keeps working.
In this example circuit, an open ground wire would read 12.27 V on both lamp terminals instead. A burned-out lamp would put the voltage across the lamp, with 0.00 V across the connector in this example circuit.
With the ground side open, no current flows, so lamp 1 drops no voltage and both of its terminals sit at the feed voltage. Nearly the whole supply voltage then stands across the open ground wire, past the lamp.
In this example circuit, an open at the connector would read 0.03 V on lamp 1 ground terminal, and so would a burned-out lamp.
The switch is in series with lamps 1 and 2, so when it stays open both of them stop. Close to the full source voltage across a switch that is turned on means it has not closed.
In this example circuit, lamp 2 is fed through the same switch, so a switch that does not close would leave lamp 2 dark too.
Lamps 1 and 2 both take their feed through the switch, so resistance in its contacts takes voltage from both of them. A healthy closed switch shows only a very small reading from slight contact resistance, so volts across it under load point to resistance in its contacts.
In this example circuit, a bad lamp ground connection would read 3.02 V at the ground connection and 0.03 V across the switch instead. With the battery disconnected and fuse 1 removed, worn contacts read 0.6 Ω across the switch in this example circuit, against 0.2 Ω healthy, a small change that the voltage drop test makes plain.
Lamps 1 and 2 share the ground connection, so it carries the current of both, and its resistance takes voltage that both lamps should get. A poor connection loses voltage, and if its drop is large the loads on it work poorly or not at all.
In this example circuit, worn switch contacts would read 3.02 V across the switch and 0.04 V at the ground connection instead. In this example circuit, a weak battery would also dim the circuit 2 lamp, and a bad lamp 1 connector would dim lamp 1 alone. With the battery disconnected and fuse 1 removed, the lamp ground connection reads 0.6 Ω in this example circuit, against 0.2 Ω healthy, while under load it takes 3.02 V, against 0.05 V healthy.
The connector is in the path of lamp 1 alone, so its resistance takes voltage from lamp 1 and leaves lamp 2 working normally.
In this example circuit, high resistance in a part that both lamps share, such as the switch or the ground connection, dims lamp 2 as well.
A fuse opens by melting under overcurrent, and a real fuse opens faster the larger the overcurrent, so a small overcurrent can take a long time. In this example circuit, a fuse 1 rated too low opens at once on the normal current of lamps 1 and 2 together, because fuses in this model have no time delay. On a real circuit, the fix is a fuse of the rating the service information gives; a fuse rated higher than that could let the wire overheat.
In this example circuit, a shorted lamp 1 feed wire reads 0.2 Ω in the same test, against OL here.
A short to ground is a low-resistance path across the source, so the current becomes very large and the fuse in its path opens. In this example circuit the short is past the switch, so fuse 1 holds while the switch is off and opens when it is turned on. The lamps are unplugged for the resistance check, because they are a second path from the wire to chassis ground and would lower the reading. On a real circuit, a new fuse opens again while the short is there, so the short is found and repaired first, and the new fuse has the rating the service information gives.
In this example circuit, a short on the lamp side of the connector and a fuse rated too low both read OL in the same test, because the feed wire itself is not shorted.
A short to ground is a low-resistance path across the source, so the fuse in its path opens. Unplugging lamp 1 takes this short out of the circuit, so the wire side of the connector no longer reaches it.
In this example circuit, a short in the feed wire itself reads 0.2 Ω in the same test.
Current flows only through a closed path from a source. In this example circuit, with fuse 1 out and the switch open, the voltage on lamp 1 comes from the circuit 2 feed wire that the lamp 1 feed wire is touching.
In this example circuit, a switch stuck closed would read 0.00 V at lamp 1 once fuse 1 is removed, because its power comes through fuse 1. In this example circuit, a feed wire shorted to ground leaves the lamps dark instead.
Current flows only if the circuit is closed, and contacts stuck together keep the lamp path closed with the switch off.
In this example circuit, a feed wire touching a powered wire keeps lamp 1 at 12.00 V even with fuse 1 removed.
A battery has internal resistance, so its terminal voltage falls as the current rises. That resistance rises as a battery runs down, and every circuit it feeds loses voltage. A no-load voltage check can read normal on a battery with high internal resistance, because almost no current flows through it, so a battery is tested under load.
In this example circuit, a fault in the circuit of lamps 1 and 2 leaves the circuit 2 lamp normal, because that lamp has its own fuse and ground connection.
The negative cable is the return path for every circuit, and a drop on the ground side counts the same as one on the feed side. In this example circuit the negative cable takes 7.94 V of the supply that every lamp shares.
In this example circuit, a bad cable between the batteries would read 7.94 V across that cable and 0.02 V on the negative cable instead. The lamp ground connection reads 0.02 V in this example circuit, so the fault is not in the lamp ground.
Every circuit takes its feed through the positive cable, so its resistance takes voltage from every lamp.
In this example circuit, a bad negative cable reads 7.94 V on the ground side instead, and a bad cable between the batteries 7.94 V across that cable.
In the 24 V view, every circuit draws its current through both batteries and the cable between them. By Ohm’s law the drop grows with the current: in this example circuit the same cable reads 3.38 V with only the circuit 2 lamp on.
In this example circuit, a bad negative cable would read 7.94 V from chassis ground to the battery negative post instead, and a bad positive cable 7.94 V from the battery positive post to the fuse block input.
The circuit is generic and does not copy the wiring of any real product. Values to compare a real circuit against come from its service information. In this model a lit lamp is a fixed resistor, while a real lamp filament’s resistance rises a great deal as it heats. A fuse here opens as soon as the steady current is above its rating, while a real fuse opens faster the larger the overcurrent, and the surrounding temperature affects it. The battery here is a fixed voltage behind a fixed internal resistance, while a real battery’s internal resistance changes with its charge, temperature, current and history. Motors are left out, because a motor draws its largest current at start and less at speed, which a fixed resistor cannot show. Apart from the lamps, which have one value lit and a lower one for the ohmmeter, every resistance in this model stays the same at any current, and electronic modules and intermittent faults are not modelled. There is no ammeter: an ammeter goes in series with the circuit, and connected across a source it acts as a short circuit that can damage the meter and the circuit. A short across a battery drives a very large current, and the heat can melt or ignite nearby material. On a real circuit, make electrical measurements only with specific instructions from a qualified person, and disconnect the battery before replacing components. 120 V AC power circuits are not covered here; where lockout is not practicable, live measurement on them is work for authorized, qualified workers following written procedures, with protective equipment chosen for the hazard, for example using CSA Z462.
Sources: open trade-training textbooks on electrical fundamentals and multimeter use, an open college physics textbook, technical documents from fuse and electrical component makers, and provincial occupational health and safety rules and guidance. Checked November 2026.
It shows how much voltage a part or a section of wiring takes while current flows through it. Voltage drop is a measure of resistance, so it finds poor connections with the circuit working. A poor connection loses voltage, and if its drop is large the load works poorly or not at all. In this example circuit, a high-resistance lamp ground connection takes 3.02 V with the lamps on, and lamps 1 and 2 go dim.
With a single open in the path, the parts that only that path uses carry no current and drop no voltage, so close to the full source voltage appears across the gap. In this example circuit, across an open lamp 1 connector reads 12.25 V with the switch on, against 0.01 V when it is connected. With an open on the feed side, the points past the open read 0 V to ground, which shows where the break is.
With the ground side open, no current flows, so the lamp drops no voltage and both of its terminals sit at the feed voltage. Close to the full source voltage then stands across the open, past the lamp. In this example circuit, lamp 1 positive and ground terminals both read 12.27 V to chassis ground with the ground wire open. With an open at the lamp 1 connector instead, this example circuit reads 0.03 V on both lamp terminals.
An ohmmeter drives its own current from an internal battery, so it is used only on circuits with the power off. Touching the probes to live parts in resistance mode can damage the meter. A path in parallel with the part lowers the reading, because parallel paths together have less resistance than the smallest of them. In this example circuit, with the battery disconnected and fuse 1 removed, the wire side of the lamp 1 connector reads 0.3 Ω to chassis ground through the two unlit lamps, and OL with both lamps unplugged.
By Ohm’s law the drop across a connection is the current through it times its resistance, so a small resistance takes more voltage as the current grows. In this example circuit, with the battery disconnected and fuse 1 removed, a faulty lamp ground connection reads 0.6 Ω, against 0.2 Ω healthy, which is the meter leads alone: a difference of less than one ohm. Carrying the current of lamps 1 and 2, the same connection takes 3.02 V in this example circuit, and both lamps go dim. A voltage drop test with the circuit working shows that loss directly.
A short circuit is a low-resistance path across the source, so the current becomes very large and the fuse in its path opens. In this simulator the battery is disconnected, fuse 1 is removed and both lamps are unplugged, then the wire side of the lamp 1 connector is measured to chassis ground. In this example circuit, the shorted lamp 1 feed wire reads 0.2 Ω that way, about what the meter leads alone read, against OL on a healthy circuit. In this example circuit, a new fuse 1 opens again as soon as it goes in with the short still there and the switch on.
It means the leads are the other way round. In this example circuit, red on chassis ground and black on the battery negative post reads 0.06 V with the lamps on, and swapping the leads reads −0.06 V.