How do most of you prefer to do EFLI tests?

[Guitarist]

Did a CU change the other day and a full range of tests. Did all my Zs using the old Ze+(R1+R2) method and all was fine. Plugged my Megger 1730 into a socket out of interest, did a no-trip live test and it came out much much higher.
So, faulty megger and dud results.
Just wondered how the majority of you guys do your EFL testing.

 

[malcolmsanford]

On design and initial verification normally by calculation

EICR would preferably be by Measurement.

 

[truckster]

By measurement. I would check your original result by linking out the rcd, the no-trip test seems to give funny measurements sometimes.

 

[JUD]

When I got my Megger calibrated in January the bloke told me that my No-Trip was more accurate than the normal test through his machine. He told me I'd be better off measuring Ze on No-Trip aswell.

 

[MarkieSparkie]

When I got my Megger calibrated in January the bloke told me that my No-Trip was more accurate than the normal test through his machine. He told me I'd be better off measuring Ze on No-Trip aswell.

In lab conditions that may be true, but add in the oxidized joints on a typical installation and result is likely to be completely different, my money is on the Hi current loop test giving the more accurate result.

 

[tony mc]

When you use the no trip the machine will only inject 15mA from line to earth to prevent the RCD tripping but on Hi loop it puts 25A across.

I am sure there are more informed members who will give a better explination but it was my interpretation that the Hi loop test would give a more accurate result!

 

[telectrix]

internal components of RCDs and MCBs can push up the measured values. in extreme cases as much as 300+ ohms.

 

[Sparky Ninja]

I contacted megger with regard to the inaccuracy of the no trip zs test last year and their response was that the noise within the test (which the reading is much more subjected to with the lower test current value) combined with the long duration of the test will result in an inaccurate value.

The advice was to refer to the individual tester for its given accuracy tolerance for the no trip test, or test it using the old "isolate the final circuit method".

 

[Guitarist]

Thanks for all the feedback guys. The Megger is fairly new so I was dubious about doing something wrong at first. After triple-checking my results, not just on the RFC but also a couple of new, short radial circuits, I realised that something was wrong. The EFL test wasn't just high, it pushed my readings into "non-conforming".
I have read also that the low-current, no-trip test isn't as accurate, but there's no point having an instrument you can't trust.

 

[richy3333]

I've had similar problems with the new 1700 series Megger too. Low reading on a TNS system was something like 3.2 ohms, but fine on the high setting

 

[richy3333]

I have read also that the low-current, no-trip test isn't as accurate, but there's no point having an instrument you can't trust.

I agree and am currently speaking to Megger about this. Send the meter back to them. The front of the meter is all colour coded for idiots. All this swapping leads around I don't think was originally envisaged by Megger because had it been then the machine would have been colour coded differently (if you know what I mean). Not sure we have a Microsoft issue - release product early and let the users/purchasers iron out the faults!

There is a software/hardware update for the 1700 series now.

 

[Guitarist]

I've had similar problems with the new 1700 series Megger too. Low reading on a TNS system was something like 3.2 ohms, but fine on the high setting

Strangely enough, I get 0.21 ohms Ze on both high and low. isswww have told me that it is definitely a fault tho and will get it sorted for me. I'm assuming that I'm not the first they have come across....

 

[Amp David]

I contacted megger with regard to the inaccuracy of the no trip zs test last year and their response was that the noise within the test (which the reading is much more subjected to with the lower test current value) combined with the long duration of the test will result in an inaccurate value.

The advice was to refer to the individual tester for its given accuracy tolerance for the no trip test, or test it using the old "isolate the final circuit method".

Same here, they wern't bothered at all and thought it as acceptable.

 

[Amp David]

I also now take measurements from both sides of the RCD and its amazing what difference n results you will actually get.

 

[richy3333]

It's a joke isn't it - acceptable - they sell some of the most expensive test equipment and think that acceptable

 

[telectrix]

it's not always the tester at fault. sometimes you can get vastly differing readings on either side of an RCD. if in doubt, i take a reading from the L busbar. it should be close to the Ze value (allowing for parallel paths).

 

[Guitarist]

it's not always the tester at fault. sometimes you can get vastly differing readings on either side of an RCD. if in doubt, i take a reading from the L busbar. it should be close to the Ze value (allowing for parallel paths).

Just tried that tel. 0.21 Ze on high current setting, 0.55 Ze on no-trip. 0.71 on no-trip at busbar....

 

[ackbarthestar]

Just tried that tel. 0.21 Ze on high current setting, 0.55 Ze on no-trip. 0.71 on no-trip at busbar....

Interesting....
So which answer would you say was the most accurate?

 

[Guitarist]

Interesting....
So which answer would you say was the most accurate?

Well, when I did exactly the same thing last week I got - 0.21 Ze on high current setting, 0.23 on no-trip setting and 0.26 on no-trip at both busbars (split-board). Hadn't thought to try all 3 again until tel suggested it. My assumption is that the meter has had a nervous breakdown from all this testing....

 

[ackbarthestar]

I personally think that if you haven't used your instrument as a 'battering ram' then its likely to be the condition of the supply, filtering and switching harmonics.
Things I might consider would be the time of day when the tests were done, what about local industry ? etc etc

Megger do recognize that inaccuracies in readings due to anomalies in the supply.

Having said that, I get odd results from time to time when doing loop testing using the non-trip function