Don asks this question, which is about measuring the system loop impedance (Zs) of an electrical circuit sited in the corrosive environment of a chemical factory. It has been answered by the NICEIC (Theme – Testing, Tools and Equipment):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: When measuring the Zs of a circuit, should I disconnect all parallel paths and measure the Zs of the circuit protective conductor (CPC) only to ascertain the disconnection time (as I was taught)?
My concern is that on our site we have metallic light fittings, which are bonded bolted directly to steelwork. If the CPC is damaged, then we still obtain a Zs reading through parallel paths.
Our site is a highly corrosive chemical site, so steelwork is always deteriorating and being altered.
Also 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.
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.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link. Experts from leading organisations provide online answers to your technical questions on a broad range of subjects. Our searchable database of existing Q & As now contains over 3,500 entries; you can browse through them here.