Electricians: measuring an single appliance's earth leakage current?

[Howardlong]

As per the subject, really.

The symptom I have is that sometimes, my central consumer unit's RCD/GFE will _sometimes_ trip when I turn on some older test equipment. Anecdotally, it does seem to be humidity related, for example if it's foggy outside.

This started happening since I moved from an apartment in the the city to a rural house, and this is the first time I've encountered it with this TE. I've had both units for ~20 years.

The TE is an HP 8753A VNA and an HP 8565A spec an.

I tried measuring the leakage at the consumer unit using a UT210E clamp meter simultaneously around both Live & Neutral, that has a resolution down to 10s of mA, but it was inconclusive, as it proved difficult to get a stable reading.

I measured the DC resistance on the two appliances, and it's up in the 10s of megohms. They do have Y caps of the order of single digit nF that I can measure.

I do have an inline RCD, but its trip current is the same as the consumer unit's central RCD.

It's kinda difficult to measure when the circuit trips.

So my question is, how best can I measure earth leakage of an appliance out of circuit? Does a PAT tester measure AC impedance for example, or is it just DC resistance?

I have both a variac and an isolation transformer if that's any good.

[themadhippy]

it does seem to be humidity related, for example if it's foggy outside.

Got any outside lights ? Maybe your installation has a fault else wear that's normally not a problem,but the additional  leakage from your test equipment is the final straw.If you've got a pat tester that does proper 500v insulation tests you could try an insulation test of the individual circuits to see if anything is amiss.

[AVGresponding]

Sounds to me like it's time to start replacing the RIFAs in the mains filters of your TE, before they let the (very smelly) magic smoke out!

Increased leakage current is a symptom of impending failure.

[Gyro]

RCDs can get old and start tripping at a lower current, especially if they have already tripped a number of times.

From your OP, it sounds as if your consumer unit has a single RCD, maybe two if it is a (slightly misnamed) 'high integrity' one. If so, you may find that the combined leakage current on that RCD from increasing number of appliances has built up to the point where there is very little headroom before it trips, i.e. your TE may be fine, but just pushing the combined leakage on the RCD over the edge. Individual RCBO consumer units tend to be better as the number of Y caps in the installation increases.

Old Kewtech KT35 (or the equivalent Robin KMP3050DL) insulation and continuity testers often come up very cheaply on ebay in the UK. They will test at 250V,500V, and 1kV and can be used to perform general insulation and appliance PAT testing (be careful where you use the 1kV setting). These will be out of official calibration, but there are no adjustment pots inside and it is very easy to verify that they are functional and within spec with a DMM and a few low and high value resistors. It's well worth picking one up for these sort of problems.

[AVGresponding]

@Gyro, yes, agreed, but remember the OP said the fault also seems to vary according to humidity; it's cracks in the RIFA resin casing that allows water in that causes the failures for these.

[themadhippy]

They will test at 250V,500V, and 1kV and can be used to perform general insulation and appliance PAT testing

A second hand pat tester maybe a better bet as not only do you  get the 500v insulation test,but also a 25A low resistance tester

[niconiconi]

I tried measuring the leakage at the consumer unit using a UT210E clamp meter simultaneously around both Live & Neutral, that has a resolution down to 10s of mA, but it was inconclusive, as it proved difficult to get a stable reading.

Special Earth-leakage current clamp meters can be purchased on AliExpress for $80. I've used one to solve a RCD tripping problem at home with great success. They're basically just normal AC current clamp with very low measurement range but high sensitivity, down to 1 mA.

Simply clamp the meter across L and N, if there's no leakage, ideally there's no net magnetic flux across the core. Any Earth leakage causes a residue common-mode current, which is measured by the meter.

First, run with all instruments on. Then disconnect them one by one while watching the meter reading. You'll find the culprit when you see a large change. I simply clamped it across the mains entry at the main electrical panel and started turning off downstream circuits one by one.

If you already have a sensitive AC current probe for lab use, just run L and N across the probe, you don't even need to buy anything (they're small and you probably cannot use it across the AC mains panel entry, of course, but you can do it at the power strip).

[mikeselectricstuff]

For individual appliances, simply break its earth connection and measure the AC current through it.
For whole installations, a high-res AC current clamp is the easiest option.

[Gyro]

They will test at 250V,500V, and 1kV and can be used to perform general insulation and appliance PAT testing

A second hand pat tester maybe a better bet as not only do you  get the 500v insulation test,but also a 25A low resistance tester

Yes, possibly so, they just seem to be more expensive / more in demand when they are listed. Personally I like the displays on the two testers I mentioned, the cheaper PAT testers tend to be a bit more go-no go. Mine gets used for general electronics insulation measurement so I find the 1kV range useful - but then I've got a 10kV one too!

[Howardlong]

@Gyro, yes, agreed, but remember the OP said the fault also seems to vary according to humidity; it's cracks in the RIFA resin casing that allows water in that causes the failures for these.

Hmm, yes, that is quite possible.

[Howardlong]

For individual appliances, simply break its earth connection and measure the AC current through it.

The problem is that it trips before I can measure it.

Any thoughts on cobbling together something with an isolation transformer and/or variac?

For whole installations, a high-res AC current clamp is the easiest option.

Yes, however I found the readings I was seeing to be unstable. Maybe I need a better meter.

[Howardlong]

it does seem to be humidity related, for example if it's foggy outside.

Got any outside lights ? Maybe your installation has a fault else wear that's normally not a problem,but the additional  leakage from your test equipment is the final straw.If you've got a pat tester that does proper 500v insulation tests you could try an insulation test of the individual circuits to see if anything is amiss.

Already I isolated some outside sockets which I believe has improved things to quite some degree, but I still get occasional trips. I'm wondering if it's accumulated leakage of several appliances and possibly the installation itself. There's no evidence of damp in the house, but it is 150+ years old.

If PAT testers measure actual AC leakage impedance then I don't mind investing on one, but if it's just DC, then I can do that without a PAT tester.

If I can measure the actual individual appliance leakage current in some way then I can have some degree of certainty: the electrician I had in just said it was the appliances, but he neither measured the system earth leakage nor the appliances. Furthermore, he seemed to think to measure system earth leakage you just measure the earth wire at the consumer unit (rather than L+N together}, thus ignoring any other possible earth paths, so I wasn't impressed.

[Howardlong]

RCDs can get old and start tripping at a lower current, especially if they have already tripped a number of times.

From your OP, it sounds as if your consumer unit has a single RCD, maybe two if it is a (slightly misnamed) 'high integrity' one. If so, you may find that the combined leakage current on that RCD from increasing number of appliances has built up to the point where there is very little headroom before it trips, i.e. your TE may be fine, but just pushing the combined leakage on the RCD over the edge. Individual RCBO consumer units tend to be better as the number of Y caps in the installation increases.

Old Kewtech KT35 (or the equivalent Robin KMP3050DL) insulation and continuity testers often come up very cheaply on ebay in the UK. They will test at 250V,500V, and 1kV and can be used to perform general insulation and appliance PAT testing (be careful where you use the 1kV setting). These will be out of official calibration, but there are no adjustment pots inside and it is very easy to verify that they are functional and within spec with a DMM and a few low and high value resistors. It's well worth picking one up for these sort of problems.

Yes it's a single RCD on the main ring only (lighting isn't protected).

Do PAT testers actually measure AC earth leakage current? Or are they DC only? I don't mind investing in one if they do. I'd much prefer actual leakage reading than pass/fail so I can see if it's a cumulative effect.

[Gyro]

No, they're DC, otherwise the leakage current [Edit: resistance] would get swamped by capacitance.

[Someone]

Do PAT testers actually measure AC earth leakage current? Or are they DC only? I don't mind investing in one if they do. I'd much prefer actual leakage reading than pass/fail so I can see if it's a cumulative effect.

Some do, the ones I'm familiar with can pass through the mains and measure the current through the earthing pin showing it in engineering units on the display. $500 ish range second hand as these things tend to be pretty long lasting in applicability/use/life.

[niconiconi]

For whole installations, a high-res AC current clamp is the easiest option.

Yes, however I found the readings I was seeing to be unstable. Maybe I need a better meter.

This $100 "CEM DT-9810 Leakage Current Tester" I got here allows me to resolve 0.3 mA and measure 1 mA. Here are some photos to show its resolution. I connected a 60 Hz sine wave from the signal generator to a 47 Ω resistor. The oscilloscope shows the voltage across the resistor, and the clamp meter shows the current. It was what I used to successfully fix my RCD problem. A similar meter can likely solve your problem too.

Before buying this meter, beware that I did a quick search and saw a negative review on Amazon.

I bought this to easily measure ground fault current in a motorhome. I'm an electrical engineer and have other ways of measuring it but wanted something more convenient such as this clamp-on meter. Using as instructed, it gave varying readings of 5 to 50 mA ground fault current, when the actual current was below 1 mA. Moving the wire pair around, or bringing the ground wire within 3 inches of the coil, changed the readings by 50% or more. I understand what's wrong with their design but unfortunately there is no way to fix it.

I never noticed this problem during my troubleshoot attempt, but it's probably somewhat suspectiable to high-frequency noise or stray field.

[themadhippy]

Do PAT testers actually measure AC earth leakage current?

more expensive models will,but as its not a requirement for pat testing you'd have to double check the particular models specs

[AVGresponding]

it does seem to be humidity related, for example if it's foggy outside.

Got any outside lights ? Maybe your installation has a fault else wear that's normally not a problem,but the additional  leakage from your test equipment is the final straw.If you've got a pat tester that does proper 500v insulation tests you could try an insulation test of the individual circuits to see if anything is amiss.

Already I isolated some outside sockets which I believe has improved things to quite some degree, but I still get occasional trips. I'm wondering if it's accumulated leakage of several appliances and possibly the installation itself. There's no evidence of damp in the house, but it is 150+ years old.

If PAT testers measure actual AC leakage impedance then I don't mind investing on one, but if it's just DC, then I can do that without a PAT tester.

If I can measure the actual individual appliance leakage current in some way then I can have some degree of certainty: the electrician I had in just said it was the appliances, but he neither measured the system earth leakage nor the appliances. Furthermore, he seemed to think to measure system earth leakage you just measure the earth wire at the consumer unit (rather than L+N together}, thus ignoring any other possible earth paths, so I wasn't impressed.

That would only give you a whole installation figure rather than an individual appliance reading. Sounds like he doesn't have the proper equipment, these things are expensive. Here's a pic of my whole house leakage, with a meter that costs the best part of £1k new. I paid, shall we say, somewhat less, as it was in need of repair. Yes, that is 1 micro-amp resolution:



As Mike suggests, break the earth (at the appliance) and measure it with a DMM, use a Safeblock or Cliffe Quicktest or similar.

[langwadt]

For individual appliances, simply break its earth connection and measure the AC current through it.

The problem is that it trips before I can measure it.

if earth is not connected  how can it trip ?

[amyk]

Break the earth, insert a (preferably logging, but one with a max function would also work) ammeter inline.

[m k]

As per the subject, really.

The symptom I have is that sometimes, my central consumer unit's RCD/GFE will _sometimes_ trip when I turn on some older test equipment. Anecdotally, it does seem to be humidity related, for example if it's foggy outside.

Maybe you should make your own meter.
MCU boards and small current modules are cheap and then you could do some longer time tracking.

But first I'd follow AVG, since finally you can't escape that anyway.

[Gyro]

Be very careful disconnecting the Earth on a live installation, it's not a recommended procedure. You have several non RCD protected circuits that, for all you know, might have high leakage currents to Earth. At least pull the fuses / MCBs on the non protected circuits.

P.S. What kind of instalation is it, eg, TN-C-S, TT etc:  https://en.wikipedia.org/wiki/Earthing_system#IEC_terminology  (IEC Terminology section).

A photo would help.

[AVGresponding]

Be very careful disconnecting the Earth on a live installation, it's not a recommended procedure. You have several non RCD protected circuits that, for all you know, might have high leakage currents to Earth. At least pull the fuses / MCBs on the non protected circuits.

P.S. What kind of instalation is it, eg, TN-C-S, TT etc:  https://en.wikipedia.org/wiki/Earthing_system#IEC_terminology  (IEC Terminology section).

A photo would help.

To clarify what I personally meant about disconnecting the CPC/earth, DO NOT disconnect ANY of the FIXED wiring, unless you are a qualified electrician. As well as the safety implications, there are issues around insurance, and if it's rented accommodation, you'll be in breach of your tenancy agreement.

What I meant is this: if the plug on the equipment is a rewireable type, remove the plug and in conjunction with a Safeblock or Quicktest, connect a DMM in series with the earth. It should not be necessary to state, but I will anyway: DO NOT do any of this while connected at the wall outlet. Once you have everything connected as it should be, only then plug the Safeblock in.

If the equipment has a moulded plug and you don't want to cut it off, you'll have to make an adapter, such as a trailing socket with the plug removed that can be connected to a Safeblock.


If you wanted something you could keep and use in the future without a Safeblock, you could make something like this:

Plug on a flex to a PLASTIC adaptable box, stuffing gland into the box. Inside the box, break out the conductors and wire them individually in flex out of the box and back in, making a small loop. Use stuffing glands. Then connect to a flex out through a stuffer to a trailing socket. With this, you can measure with a clamp the individual currents on each conductor, and all the exposed wiring is double insulated, so is reasonably safe.
DO NOT use a metal adaptable box, it will effectively become a shorted transformer winding.

[Kean]

Measuring leakage through the earth wire may not tell the full story unless the product is double insulated and not connected to anything else.
As nico suggested above, you should use a leakage clamp meter to measure both L & N wires together and see if there is any imbalance.
Uni-T make the UT251A AC clamp meter which has 1uA resolution for smaller currents, but can read up to 60A.

[Howardlong]

Measuring leakage through the earth wire may not tell the full story unless the product is double insulated and not connected to anything else.

Correct, that is my understanding. I was quite taken aback when the electrician discounted my measuring of L&N together in the clamp in favour of just measuring the earth at the consumer unit. Measuring just the earth at the CU isn't taking into account any other possible paths to earth. Furthermore, the RCD measures L&N together, and that's what I'm trying to fault find.

As nico suggested above, you should use a leakage clamp meter to measure both L & N wires together and see if there is any imbalance.

That is what I have been doing but the meter I have (UT210E) gives unstable results.

Uni-T make the UT251A AC clamp meter which has 1uA resolution for smaller currents, but can read up to 60A.

OK, I'll see if I can source one.