Q & A of the Day – In order to find the disconnection time, should I disconnect parallel paths and measure the CPC Zs only?

 

Question: I’m going to measure the system loop impedance (Zs) of a circuit. Must I disconnect all parallel paths and measure the Zs of the circuit protective conductor (CPC) to find out the disconnection time?

 

My worry is that on the site concerned, there are metallic light fittings that are bonded bolted directly to steelwork. If the CPC is damaged, then we still obtain a Zs reading through parallel paths.

 

Note that this site has a highly corrosive chemical atmosphere, so steelwork is always deteriorating and being changed or repaired.

 

In addition, one of my concerns is, if our earth (parallel) path is through temporary scaffolding and could be removed, this could possibly leave our equipment with no earth protection. Am I right to worry about this?

 

Answer: It is advisable to obtain the R2 reading by dead test and as long as it is confirmed that the condition of the CPC in the wiring is adequate.

 

If this is all in place, then upon energisation, a Zs can be obtained safely to confirm that any calculated Zs value from the results of R1+R2 +Ze is actually equal to or less than the calculated value.

 

If the measured reading comes back less than this, then it would indicate that parallel paths were in place when testing. This doesn’t make it unsafe. You have to ensure that the CPC of the circuit is able to carry fault currents of the circuit and in safe condition to do so. Then any parallel path that may exist does not matter as the CPC is doing what it is intended to do so without any additional risks being in place.