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Posted By Topic: Earth continuity testing (& tester)

DougP
Jan 09 2018 13:38

Doing main earth conductors, or measuring other earth conductors.. or non-live loop impedance measurements.

I thought there was something in some standard, about requiring 200mA test current?
But I don't know where I saw it. Maybe IEC61557?

I know that most multimeters are certainly incapable of measuring accurately under 1ohm (especially a 0.1 resolution meter with a +/-2 or more, count accuracy for example).

I have a meter with a 0.01 resolution and +3 digit accuracy up to 20ohm, but is there a requirement to use equipment this accurate?
Otherwise, how are most electricians doing their measurements?
   

pluto
Jan 09 2018 15:33

Many use (and I do) use the low ohm scale of my
Megger to get low ohmic values with a reasonable level of cretainly. A Digital multi meter is NOT satisfactory if can NOT give the required level of accurancy.
   

DougP
Jan 09 2018 18:58

Yep, there are quite a few older meters that did a reasonable low ohm scale.

But some people are raving about the Fluke 1587FC insulation tester..

But its lowest ohm scale is only 600ohm, with 0.1 Ω resolution and ±(0.9% + 2) only about 1mA test current.

Not really acceptable for a top of the line meter.
   

AlecK
Jan 10 2018 10:12

that's a small error, but not small enough given the resolution. Basically a meter resolving to 0.1 is never going to be suitable for measuring earth continuity (or EFLI); because while the percentage error has little effect the "digit" error is very large compared to the value being measured.
The actual value is somewhere in a range of 0.4 ohms - and you can't know where in the range the actual value lies. So a reading of 0.4 could be anywhere from 0.2 to 0.6.

The "low ohm" range on most IR testers resolves to 0.01 ohm, so a "digit" error of +/- 2 digits is a range of 0.04, and even a crappy accuracy with 5 digit error is still only a range of 0.1.

ESRs require testing of PEW to be i.a.w. "3000"; and "3000" cites (but does not mandate) the test methods of "3017". Neither of these Standards sets any particular accuracy requirement for test equipment used for earth continuity; but using any meter with an error range in the same order as the values being measured can't be claimed as acceptable.
   

DougP
Jan 10 2018 12:47

3017 does say "All test equipment should be suitable for its intended purpose".

But I'm sure there are electricians that just don't realise that their average multimeter just isn't suitable. Even my very expensive industrial multimeter only has 0.1 resolution, but with a +/-1 count, it could nearly be used under 0.5ohm.

But my installation tester has a 0.01 resolution, with a +/-3 digit accuracy. So it should read within 0.03 ohms.

My original reason for posting, was more about where I thought I had seen the 200mA requirement for low resistance measurement - if anyone knows a reference to that?
   

pluto
Jan 10 2018 18:04

DougP Jan 10 2018 12:47

Your question
My original reason for posting, was more about where I thought I had seen the 200mA requirement for low resistance measurement - if anyone knows a reference to that?

My answer
I have a copy of IEC 61557-4 (1997) now superseded by a 2007 edition, but would expect very little change to have occured in the new edition.
Test equipment: Mesurement of earth conductors and equipotenial bonds.

Max open circuit voltage 24 volts and NOT less than 4 volts.

Measuring current not less than 0.2 amps (200 mA) over the range of 0.2 ohms to 2 ohms.

I recalled where I had seen the test current in a IEC test instrument standard. Details above.




   

mowgli
Jan 10 2018 20:48

My Megger MFT1835 meets those IEC numbers. For RLO it includes a user selectable test current, 200/15mA.

Under what test conditions should the low current be preferred?
   

pluto
Jan 10 2018 22:40

I have never seen a atatement why the low current is available by a manual switch; the only thing I can think of would be to save battery energy during fault finding purposes and only use the higher current for resistance measurements for certification purposes.
   

pluto
Jan 10 2018 22:53

another reason the 20 mA setting should not trip an RCD.
   

AlecK
Jan 11 2018 09:01

I don't believe it has anything whatsoever to do with RCDs, because earth continuity testing is done on de-energised circuits; and even if still energised, RCDs don't react to current in the PEC - they react to current imbalance between A(s)& N.

The lower test current may well save battery power, but I suspect it has more to do with measuring non-power circuits without letting the smoke out of delicate components.

I've often heard it suggested that continuity should be tested at the highest available test current, to "blow apart" any bad connections and ensure the PEC can carry fault currents. But actual fault currents are so many times greater that any selected value seems arbitrary. On the other hand, a tester capable of generating currents of several hundred amps would be impracticable in the field.

Would be interesting to see the documentation that underlies these test parameters. The reasons behind "rules" often get lost over time; partly because the purpose of Standards is to set the rules, not to educate the users. I'd like to see next revision of "3017" include more guidance on test instruments. Currently it cites IEC 61557, but since almost no sparky has access to that it just gets ignored.
   

Sarmajor
Jan 11 2018 19:34

I use the low current setting to test general circuit continuity in installations that have RCD protection.
It is also very handy for doing testing on caravans.
I use the high current range to test the continuity of the earthing lead from the plug to each socket outlets earth connection.

I also test the continuity of the phase and neutral on the high current range from the plug to a socket outlet and this will usually trip the RCD confirming that it provides protection even when the circuit is not energised. (Which I realise is only a requirement of portable RCD's but many fixed units will also do this as well.)

On vehicles where the RCD provides protection when not energised then I use the low current range to test the phase and neutral from the plug to each socket outlets phase and neutral connection.
This means that the RCD will not trip and I can operate the switch to confirm that the switch is operating in the live conductor and the polarity is correct. Of course some caravans have socket outlets with double pole switching but that is OK by me.
   

pluto
Jan 11 2018 19:55

Sarmajor Jan 11 2018 19:34

Thanks
I was about to write up how using the low test current for continuity testing on a Megger is used when an RCD is part of the test circuit

You and I think the same way on use of the Megger low test current facility for continuity testing is the same. Prehaps we read the same text book!


   

mowgli
Jan 11 2018 20:18

Pluto, AlecK, Sarmajor, thank you gentlemen. That's mint!
   

DougP
Jan 11 2018 21:14

Thanks for all the comments.

Pluto, those specs from 61557 match my meter specifications.
Up to 7.5ohm the test is >4V open circuit voltage and 210mA test current with 0.01ohm resolution.

Obviously according to ohms law, the higher test current must give a more accurate reading at lower resistances. So when measuring resistances way under 0.5ohm, the higher (200mA) current is necessary.

As for the lower current test option, my tester doesn't have a manual setting, but the current does reduce as the auto range scale increases. 210mA @ 7.5ohm, 100mA @ 35ohm, 20mA @ 240ohm, 2mA @ 2000ohm.

If I wanted to measure resistances in more sensitive equipment (instead of lengths of copper wire), and at much lower current, I would normally use my multimeter, which is plenty accurate at around 0.2ohm margin of error.
   

AlecK
Jan 12 2018 08:51

Yes a sufficiently high continuity test current could trip an RCD if applied through either A or N, thus creating an imbalance. But unless it is set at within 5% of rated residual current, it's not a valid test of the operation of the RCD.


BTW, continuing to provide protection when supply voltage lost is not actually a requirement even for portable RCDs, most of which have 0-volt drop-out instead.

None of which has anything to do with testing earth continuity (the issue under discussion).

For those with meters having multiple test current settings, it would seem logical to use the higher test current for earth continuity.
   

pluto
Jan 12 2018 09:24

Alac K

You may wish to read ESR 2010 regulation 24 (2) (b) again.

Some portable RCDs use switchboard mounting type RCDs which do not trip on the loss of the supply voltage, the tripping is done by induced current in the sensing coil inside the RCD by the imbalance of the currents flowing in the live conductors.
   

DougP
Jan 12 2018 09:51

I have tested switchboard type RCDs without power (230AC)applied, and they do continue to provide protection. In fact I tested and tripped one using only a flat button cell. Obviously the cell still had enough power for ~30mA.
   

AlecK
Jan 12 2018 19:18

My point was that portable RCDs need only comply with one of the two options. Most use 0-volt drop-out, some continue to protect, but either is a compliant option.