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Posted By Topic: EFLI Test With Overload in Circuit

Cripp
Mar 02 2018 08:10

Firstly, is a EFLI test a legal requirement for fixed wired appliances?
If so how would I go about testing EFLI when an overload is located in the circuit?

I have done earth loop tests through overloads but the results are always above acceptable levels. Cheers
   

AlecK
Mar 02 2018 09:32

An EFLI test is not required by Section 8 of "3000" for new circuits supplying directly-connected equipment only. The requirement is to test circuits that supply socket outlets that are not protected by an RCD.

But there are several related requirements.

Where automatic disconnection is the adopted method of providing fault protection [2.4], we have to ensure for ALL circuits that the circuit protective device (fuse,mcb, or RCD)will trip within the time set by 1.5.5.3 & 5.7.

The earth continuity test provides some of the assurance we need, as does the much longer time permitted for non-socket circuits compared to sockets.


When you say the results you've had with an overload in circuit are unacceptable, it's likely that either there's something very wrong with your installation, or you're not testing correctly, or you're not using the correct value for "acceptable". After all, the overload function of an mcb operates on exactly the same principle as a thermal overload for a motor - a resistance causes heating to trigger the tripping function. So it's not the presence of a relatively small impedance caused by an overload that is upsetting the readings.

To be clear, neither the mcb's thermal function, nor the overload's thermal function relates to fault protection, just making the point that mcbs introduce impedance in exactly the same way as overloads. The fault protection function is done using the short-circuit magnetic trip of the mcb.

While an EFLI test is only required for circuits supplying sockets, there's no reason to avoid testing other circuits; using the same methods as described in 8.3.9.2 of "3000".(also in "3017". However you'll need to do some maths because Table 8.2 doesn't provide any values for 5-second trip times. Table 8.1 does, but only for HRC fuses.

We can't just assume that the short-circuit function will take care of it, because the specified trip times are different, and because PECs in the EF loop are often smaller (more impedance) than other phases or the N used for short-circuit trip calcs.

But we should know enough other facts that we can draw some conclusions. First we know - from earth continuity testing - that the PEC side of things is of low enough impedance to trip within 0.4 sec. We know the required trip time for non-sockets is 5 seconds. And we should know that the actives of the circuit are of low enough impedance to comply with volt drop requirements. That's the same impedance that influences trip time for fault protection; so if there's a problem it's going to show up long before there's an earth fault. And we can reasonably rely on the rest of the EF loop, excluding the final subcircuit, being OK. The assumptions we make for the part of the EF loop external to the installation are explained in Notes to table 8.2 and in detail in Appendix B.

Taken overall, these factors mean we can be confident of EFLI being compliant for non-socket circuits without full testing.

For RCD-protected sockets, even easier: the max trip time is 0.4 seconds; and the RCD will operate within 0.3 seconds. Job done.

There's also the safety side - live EFLI testing for anything other than a socket is dangerous. Not only do we have to access live terminals of the equipment at the end of the subcircuit, but that equipment could be motor-driven equipment that will likely be in motion while we're attempting to connect our test gear to the motor.

Since HSW rules, including "4836" (which ESR 100 requires us to follow) mean we're not allowed to do anything "live" that we don't absolutely have to do "live", we should be using the "dead" loop resistance test method whenever possible,; ie test BEFORE connecting the subcircuit to supply at the switchboard.


   

Cripp
Mar 02 2018 11:21

Thankyou for clearing that up for me AlecK.
The impedance of the overloads is quite high(have tried the live test on multiple overload brands)thus the unacceptable readings. With the O/Ls taken out of the circuits ELFI results pass, but altering the circuit seems to null and void the test.

Your's and others wealth of knowledge on this forum is incredible, thankyou for sharing it with us AlecK.
   

DougP
Mar 02 2018 14:51

Cripp.
What are you meaning by overload?
A normal MCB or RCD in the circuit has negligible resistance that would affect a loop impedance test.
   

mf51to1
Mar 02 2018 20:29

[quote]I have done earth loop tests through overloads but the results are always above acceptable levels. Cheers[/quote]

I have found this to be the case testing EFLI on C-bus controlled lighting circuits too.