Home Electrical circuit testing continue…

Using a meter

To test for an open circuit in a component, make a note of and then remove the original wiring to that component. If this is not done, false readings may be given from other items that may be in circuit. Attach the ends of the two probes of the meter to the suspect component. For example, to test a heater for continuity, place the probes on the tags at the end of the heater and watch the meter. The needle should move. At this stage, it does not matter if the needle does not reach zero. If the heater is open circuit (no movement), it should be tested further. If closed circuit, the heater has not failed.

Leap-frog testing

Often the most effective way to trace a fault is to use a simple but logical approach. One such approach is called the leap-frog method and can be used to find the failed/open circuit part or parts.

Let us assume that the appliance does not work at all, so we cannot deduce where the problem lies purely from the symptoms. A quick check of the supply socket by plugging in another appliance known to be working will verify (or not) that there is power up to that point. This confirms that the fault lies somewhere in the appliance, its flex or plug. We know that during normal conditions, power flows in through the live pin on the plug, through the appliance (when switched on) and returns via the neutral pin on the plug. The fact that the appliance will not work at all when plugged in and switched on indicates that an open circuit exists somewhere along this normal live-to-neutral circuit.

Leap-frog testing using a continuity meter works as follows. First, test that the meter is working correctly; touch the test probes together and the meter should indicate continuity. Connect one probe to the live pin of the appliance’s plug and the other on the live conductor connecting point in the plug. Continuity will be found when the pin, fuse and their connections are all right. If this check proves no fault, move the probe from the live conductor point in the plug to the live conductor connection in the terminal block within the appliance. Continuity should be found; if not, a fault between plug and terminal block is indicated.

When continuity testing flex, move the wire continuously, bending it back and forth along the entire length, so that wire breakage internally — which often causes intermittent continuity — can be completely checked. If this test is all right, move the probe to the next convenient point along the live conductor, in this instance, the supply side of the on/off switch. Again, continuity is required. An open circuit indicates a fault between terminal block and switch connection.

The next step is to move the probe to the opposite terminal of the switch. Operate the switch to verify correct action (on continuity, off open circuit).

If this test is all right, proceed to the next point along the wire, in this instance a motor connection. Again continuity is required. If it is all right, move the probe to the other terminal of the motor. This, once again, should indicate continuity of circuit through the motor.

At this point we will assume that an open circuit has been indicated, so go back to the last test point and verify continuity up to that point. If it is found to be all right, then a fault has been traced that lies within the motor and a similar leap-frog test can now be made of the individual parts of the motor — field coil, brushes, etc. The fault, once accurately located, can then be easily repaired.

This simple, methodical approach is all that is required to find such problems. It is best to break down more complex circuits into individual sections, such as motor, heater and switch, and test continuity of each section from live through the individual parts and back to neutral. This may involve moving the live probe that would normally remain on the plug live pin to a more convenient supply point within the appliance to avoid misleading continuity readings. With practice, faults can be found even in complex wiring this way.

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