Came across an interesting case recently, whereby the engineers taking care of a particular shopping mall had failed to obtain an earth loop impedance value on just one of the phases. Issue came to light as the electrical inspectors failed to obtain a complete earth loop results for the new tenants that are setting up shop in the mall.
Was called in to investigate if power quality is a factor here.
Background
– Similar “incomplete” earth loop values were obtained at the LV Main Switchboard. “OL” when measured between Phase L1 and Earth. Had used different sets of earth loop testers.
– Thorough checks were conducted on the distribution transformer, earth cables and the neutral-ground connection at the transformer.
– Loads on this particular switchboard include the building’s chiller systems and some tenants’ loads.
– Was reported normal earth loop values could be obtained when the chiller system is not in operation.
Recall: Earth Loop Impedance Test
– One of the tests conducted for an Electrical Installation “Pass” Certificate.
– To ensure when a fault occurs in an electrical installation, sufficient current will flow to operate the fuse or circuit breaker protecting the faulty circuit within a pre-determined time.
Recall: Earth Loop Tester
– Operates by inducing a current from the Supply system, by introducing a calibrated load between the phase conductor and the protective earth.
– And then monitors the voltage difference.
– Comes in various forms (Multifunction tester vs. Dedicated tester); (3-wire types L+N+E vs 2-wire types L+E)
Findings
– Conducted separate earth loop tests at LV Main Switchboard and monitored the quality of supply.
– Apart from one failed attempt via the multifunction tester, subsequent tests produced repeatable earth loop impedance values across all the 3 phases.
– “OL” when measured between Phase L1 and Earth, used “K-brand” earth loop tester(s).
– VTHD on “high side” but expected due to the a few nos of variable speed drives being used at the Chiller system.
– Phase L1 – noticed multiple zero crossings.
Conclusion
– The quality of the supply waveform on Phase L1 affected / influenced the operability of the other earth loop testers, resulting in giving “OL” readings. An interesting case!
– Newer earth loop testers do have a ‘harmonic component’ – to cater for (IEC 61557-3:2007).