Could somebody explain please

[mhar]

Went to some family friends earlier who are having nuisance rcd tripping problems.
They have just about every probable cause going - rusty freezer in a garage fed underground in non swa, underground feed to a shed in non swa, enough fridges, freezers and washing machines to put Currys to shame etc etc but a feeling that it is the oil fired central heating that is causing the problem, mainly because the frequency has been increasing since the end of summer.
I switched every mcb on the board to off and then tested the rcd on two seperate circuits. Test 1 was on a circuit that had the boiler, a dishwasher, a fridge freezer and a pump to fill a header tank with harvested water for toilet flushing. I was unable/unwilling to ir test this circuit as I didn't know what else may be on it and couldn't get to any isolators etc. The second test was using another rfc but with nothing plugged in and the 3 ir results were all 299+ MOhms.
Test 1 Test 2
.5 0 1999+ 1999+
.5 180 1999+ 1999+
1 0 300+ 300+
1 180 47 26
5 0 22 26
5 180 22 21mS

Ramp 21 24mA

I don't need a conclusion but an explanation! I have got out my pictures of how an rcd works but cannot see how it can be more sensitive on 1 cycle compared to another. Additionally I cannot see why a circuit which undoubtedly had some earth leakage on it already before my meter introduced more would take longer to trip (result 1 180deg) than on a completely clear circuit.
At the moment I am tending towards having a few beers to help me sleep unless somebody could explain please! Sorry if this is a basic question

 

[SirKit Breaker]

This could be earth leakage in excess of 30mA due to the state of the apppliances. Try to borrow a PAT tester that measures earth leakage, test each appliance, and add up the total leakage.

Cheers...............Howard

 

[mhar]

Sorry, when I typed the above table it looked quite clear and neat, when I posted it all got bunched up and is now difficult to read.
My lack of understanding is twofold 1) Why does an rcd behave differently on 0 and 180deg - it obviously does hence why rcd tests are at top and bottom of cycle, and 2) I would have expected test 2 on a clear circuit to have reacted slower than test 1 on a circuit which already had an element of imbalance

 

[GLENNSPARK]

maybe because a fault reaches full potential on the first quarter cycle...

 

[i=p/u]

thats why you only realy need to to test 180@

 

[Goody]

If you strongly suspect oil fired central heating boiler as the culprit, why not connect it on a circuit on another RCD using an extension lead or some sort, if possible and play the game of elimination. Good luck to you!

 

[mhar]

Sorry Goody, don't see where that would get me because of lack of discrimination.
Anyway, that is not really the question, the tripping just led to the question, but thanks anyway!

 

[telectrix]

an earth leakage clamp meter will solve the problem. unplug or disconnect all appliances first. see if there's any leakage, then reconnect appliances in turn until you find the one that bumps up the leakage.

 

[Marvo]

Many RCD's use electronics to amplify the fault and to help operate the tripping mechanism. This electronic arrangement is sensitive to the imbalance between live and neutral caused by a leakage current to ground or earth. Problem is the electronics often only works on either the positive or the negative half cycle.

When you test an RCD the tester generates an AC waveform that starts on the positive half cycle and only goes negative 10 milliseconds later. If a particular RCD requires the full 10miliseconds of positive waveform to actuate the tripping solenoid plus 30 milliseconds for mechanical lag to complete the electrical isolation then it would trip in 40milliseconds.

BUT if you start the test with a waveform that's 180 degrees out of phase so it starts going into the negative cycle first. The same RCD would only start sensing a fault current after the first negative half cycle had finished. This would effectively add 10 milliseconds to the test result because it would still take 40 milliseconds to trip plus the first negative half cycle that it didn't detect makes 50 milliseconds.

When testing an RCD you want to know the worst possible tripping time under fault conditions.

Confused? Maybe someone with better English terminology can clarify better.

 

[mhar]

Making sense now, thanks for the explanation. Still going to have those beers though!
Would this explain the second question as well? The fact that the test on a 'clear' circuit disconnected quicker is more chance as in it just happened to start the test at the 'wrong' time of the cycle OR the tester was not generating the test current in phase with the appliances on that circuit, therefore their leakage was irrelevant (to the test)?
Sorry to go on but these things bug me! I know it won't help wall chasing at all!

 

[Marvo]

The testing I outlined was for an RCD that's off-line. I only have a bench tester for RCD's and that's how it works. I'm not sure how the more modern testers perform their on-line tests, on-line and ramp testing isn't a legislated requirement in my country.....yet, we just test with the button on the unit itself.

I'm sure one of the UK guys who has a more modern type of tester can throw more light on your questions.

 

[MarkieSparkie]

Test 1 Test 2
.5 0 1999+ 1999+
.5 180 1999+ 1999+
1 0 300+ 300+
1 180 47 26
5 0 22 26
5 180 22 21mS

Ramp 21 24mA

RCD testing a loaded circuit is likely to produce unreliable timings. The load(s) on the circuit also loads and distorts the test pulse generated by the RCD tester which may have an effect on the accuracy of the timing.
From the results you have posted you appear to have a BS7671 non-compliant RCD, failing at X1 IΔn, as the device should trip within 300mS on both half cycles for a BS EN 61008 device or within 200mS for a BS 4293 device.

 

[Engineer54]

It is worth noting, that all RCD testing is carried out at the load terminals of the RCD device, an not at let's say socket outlets. You are basically testing the RCD device itself and not the overall circuit. Some manufacturers advice you test via short tails from the load terminals, but generally the manufacturers advise, that all testing of there units should be at the load terminals with the load circuit cables disconnected (to remove any latent leakage that may affect your test results).