EFLI test

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Hi

Anybody who does these tests regularly...

I was assessed by my firm today (passed with flying colours!!) but the guy asked me what the implications of running a Zs test were. IE what precautions should you make and what things should you check before running one?

As a teaser, is anybody willing to put their neck on the line and put forward any ideas?

I'll nip back later....
 
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you should carry out efli only after inspection and continuity testing have confirmed, so far as is reasonably practicable that all cpc's/ main and supplementary bonding conductors are present and correct.

if not there is a chance that exposed conductive/extraneous conductive parts may become live during the test.
 
What he said ^^^

Although better to do an ELFI test recklessly than to do without, if there is a dodgy rotted off earth, its better to have this cause a dangerous situation during an ELFI test (with it being a controlled duration, etc), rather than later down the line when you didn't know there was a problem because you didn't test...
 
Yeah, basically he said that some testers shove 23A down the line when testing, and if your bonding is not up to scratch & old Mrs Timmins is washing up, she could end up a bit frazzled...

I didn't realise this. We use Alphateks, so I rang the tech guy up and asked him what current levels their kit uses. He told me 2.5A on the ordinary test setting & 0.5A on the soft test.

Bit of a difference...but he said we should still be wary of doing the test, and if we can't verify MEB's (eg water pipe concealed), then all household members should be restrained away from metallic objects.

As I'm the technical guy for our area, I feel a presentation coming on...!
 
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If re-testing an existing installation then it must be made sure that there is nothing that could provide a parallel earth path eg testing EFLI on ring circuit, boiler is wired in via FCU on ring, bonding on pipes provides parallel path giving falsely low reading ( boiler could be ripped out next month)
 
I'm not sure how many people know how an ELFI tester works and how many don't etc, but I expect its probably relevant here.:

Basically it creates a fault to earth through a resister (23A is a common figure), it measures the supply voltage with the fault there and without it there and sees how much the voltage sinks when it is there, ohms low can then be applied to find supply impedance.

It doesn't work perfectly:

1) As has been said, if your earth is missing, dangerous voltages will appear on metalwork, but as the tester is doing it for controlled durations, etc its likely to be less dangerous than if a real fault to earth occured

2) also the way of measureing it isn't perfect, other loads on the DNOs LV system stop you getting a perfect result (if you are feeling technical then: http://www.theiet.org/Forums/forum/...=5&threadid=2600&highlight_key=y&keyword1=30% may be worth a read), the instrument accuracy is not that that amazing either

3) The 23A test will trip RCDs (and some B6 breakers) the solution to this is a much smaller current test in most machines on 'non trip' setting(although some do 'jam' the RCD with DC currents), the smaller the test the less accurate the result will be, in the case of RCDs the saving grace is Zs arn't quite as critical when an RCD is fitted, better to use a 23A test when no RCD though, also if you have a dodgy earth held on my not much, 23A might blow it clear and show up the problem where a smaller current will not, the opposite is true, it might fix a dodgy connection, if there is high resistance connection due to oxidation, a 23A test might clear it up, while a lesser current would not

4) The accuracy also drops off as the Zs decreases, basically the smaller voltage change the unit measures, the lower the accuracy ( http://www.theiet.org/forums/forum/...eadid=12820&highlight_key=y&keyword1=accuracy )
 
Yeah, some testers saturate the RCD's coil to prevent tripping.

But why is the Alphatek's current so much lower than 23A?
 
Probably some trade off in design of the tester, the higher the current the harder its going to be on the teser both electrically and thermally, make the current smaller and you can cut costs...
 
When playing with my tester I have carried out a soft test on an RCD protected circuit to prevent the RCD from tripping, then carried out a standard EFLI test on the same circuit with the RCD bridged and got two quite different results.
 
RF Lighting said:
When playing with my tester I have carried out a soft test on an RCD protected circuit to prevent the RCD from tripping, then carried out a standard EFLI test on the same circuit with the RCD bridged and got two quite different results.

Which one was higher, ooi?
 
RF Lighting said:
...carried out a standard EFLI test on the same circuit with the RCD bridged...

Tut, tut, tut, young man.. the Testing Police will be round later to rap your knuckles! :D
 
Always is in my experience.

If I do a soft-test on a non-RCD supply, it is higher than an ordinary test result.
 
securespark said:
Always is in my experience.

If I do a soft-test on a non-RCD supply, it is higher than an ordinary test result.

Yeah, and would be a whole lot less faffing about than linking out RCDs :oops:
 
Indeed. Our first test once inside a customer's property is an EFLI test. In fact, the lazier of us don't bother seeking out the mains, and just run a soft test regardless!
 

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