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Can I perform an Earth Loop Impedance test with an iLight system?

When installations are Earth Loop Impedance tested using electronic earth loop impedance testers and the system includes Inductive source controllers (SCI type) the results that are obtained appear to fall outside of the requirements of BS7671.

Why is the reading on the earth loop tester so high?
 
The diagram below shows a simplified version of the live circuit of an SCi/SCMi type Source Controller.
 
 
The reason why the reading is high is due to the inductive nature of the choke in series with the load circuit.
 
Most Earth Loop Impedance testers introduce a high current in the system for a short period of time and make their measurements during this time. This is usually carried out by placing a load from the phase to the protective conductor for a short period (typically 40ms) during which time the current rises to 20A/25A and the impedance is calculated by dividing this current by the supply voltage. The resistance of the added resistor is subtracted from this calculated value before the result is displayed.
 
An alternative method is to measure the supply voltage both before and whilst the loop current is flowing. The difference is the volt drop in the loop due to the current, and loop impedance is calculated from voltage difference divided by current. The testing time is long enough to make the measurement but short enough not to allow the protection device to trip.
 
Both methods cause a rapid rise in current that the coil in the circuit resists, as it is designed to do during normal operation.
 
Why is the coil in the circuit?
 
The coil is placed in the circuit to reduce the electrical noise caused by the fast transients as the triac turns on during each half cycle of the mains. The coil has the effect of introducing impedance to smooth the edges of these transients as the current rises rapidly. Without the coil the source controller would operate but would fail to meet EMC requirements and cause interference with other electrical equipment, audio systems etc.
 
If the coil were not fitted the earth loop impedance measurement would be, provided that the other parts of the circuit were correct, within the requirements of BS7671.
 
How can impedance for the circuit be determined?
 
The easiest way to test the circuit is to temporarily link out the triac and coil of the source controller as shown in the diagram below. Note that the cable used for the temporary link must be capable of carrying the full load of the circuit. This will not harm the source controller even if the triac is turned off by the control electronics at the time.
 
 
An earth loop impedance test can then be carried out in the normal way using ANY suitable Earth Loop Impedance tester.
 
This inability of triac dimmer circuits to be earth loop impedance tested will be common to all circuits of this type regardless of manufacturer.
 
Other types of source controller are not affected in the same manner. High frequency ballast controllers such as the SCH1210, SCH0410 and switching controllers such as the SCS1210 are not fitted with coils, as they do not switch the mains waveform other than between full on and off using a contactor.
 
If you require any further assistance with regard to this issue please contact iLight Technical Support.