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Topic Title: RCD problem
Topic Summary: Ohms law
Created On: 17 January 2014 08:37 PM
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 17 January 2014 08:37 PM
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webbix

Posts: 133
Joined: 29 October 2006

am I right in assuming that using ohms law on an RCd 30mA if I ramp test it and it trips at say 26mA it will need an resistance on the circuit of 9230ohms or less to trip the rcd

Problem I got is a RCd trips on a kitchen ring every 2-3 days always in the middle of the night. after testing I get L-N 26meg, L-E19meg N-E 16meg @ 500v

my head hurts any advice much appreciated

john
 17 January 2014 08:46 PM
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daveparry1

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None of those ins. readings would be low enough to trip a 30m/a rcd.
 17 January 2014 08:55 PM
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Grumpy

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None of those readings should be low enough to trip an RCD. In theory. In practice, I think you should be asking yourself why the readings are so low on a kitchen ring.
 17 January 2014 09:02 PM
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sparkingchip

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Disconnect the RCD and ramp test it, then reconnect the circuit and repeat the ramp test, subtract the second reading from the first and you will have the leakage of the circuit in milliamps if any.

As Dave said you shouldn't have a trip with those insulation test results, so that leaves you looking for an elusive intermittent fault.

Andy
 17 January 2014 09:08 PM
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Grumpy

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This is a kitchen ring. Presumably it's not huge. If you had just installed this ring and got those results you would be horrified. Something is not right. Check out the ring before chasing elusive faults.
 17 January 2014 09:51 PM
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sparkingchip

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Fair comment Grumpy,

Hmmm. I've been reprimanded, so I will amend things.

Whilst undertaking the inspection and testing course some years ago we were advised as to how to use the insulation tester to check for damp and drive it off, under controlled circumstances.

I assume all electricians understand the process and the precautions required, which are the same as for any electrical testing regime.

Edited: 18 January 2014 at 12:01 AM by sparkingchip
 17 January 2014 09:52 PM
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leckie

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Why are you looking at the fixed wiring?

Have a look at the loads, appliances. That more likely to be the issue.
 17 January 2014 09:55 PM
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Grumpy

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What? If you're insulation testing then surely you will have disconnected any connected loads?
 17 January 2014 10:04 PM
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sparkingchip

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Last Wednesday I told the customer that the washing machine I had missed in the out building when I unplugged appliances wouldn't pass a portable appliance test.
 17 January 2014 10:04 PM
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leckie

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Grumpy
Err, yes! That's why the insulation readings might be too high to trip an rcd. But when the appliances are connected, as I assume they are from reading the post, and you get a tripping rcd, it might be an appliance that is causing the problem.
 17 January 2014 10:23 PM
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redtoblackblewtopieces

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First thing to do is find out what's different in the installation at night time if this is when it trips as surley during the day more standard leakage would be applied to the circuit - basics
Kevin

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Compliance by Approved Documents
 17 January 2014 10:26 PM
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leckie

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Electricians on this site, a tip. Buy a clamp meter that can measure in milliamperes. Well worth it.

Edited because I might have been a bit insulting
 17 January 2014 10:45 PM
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Jaymack

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Originally posted by: webbix

Test for any standing earth leakage with no appliances plugged in, then PAT those appliances with earth connections. Appliances with heating elements are always suspect.

Problem I got is a RCd trips on a kitchen ring every 2-3 days always in the middle of the night.

It could be just that the voltage is higher during the night, due to reduced loading on the network.

Regards
 17 January 2014 10:53 PM
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leckie

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Well done Jaymack. Now that really is a practical demonstration of ohms law.
 17 January 2014 10:56 PM
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mabx

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Is the central heating fed from that RCD? I'm thinking heat+water, cools down overnight - condensation, then at some early hour (= middle of the night when everyone's still asleep in bed) the timer clicks on and pop goes the rcd.

just a though.
 17 January 2014 11:21 PM
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leckie

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Doubt it.
 18 January 2014 12:06 AM
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sparkingchip

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I've edited my earlier post.
 18 January 2014 02:50 PM
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Banners

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Originally posted by: sparkingchip

Last Wednesday I told the customer that the washing machine I had missed in the out building when I unplugged appliances wouldn't pass a portable appliance test.


Rather you than me, sparkingchip. a washing machine would be damned heavy to carry around!
 18 January 2014 03:06 PM
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leckie

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I use my clamp meter in the milliamp range for a bit of quick guidance. You can hook it around the L & N supply to the RCD and check for leakage with appliances on, appliance off, or even different MCB's on or off and note down the differences. Individual appliances can be check for earth leakage also. I have a 1 metre 13a extension lead with the outer sheath removed for a short section, enough to fit the clamp meter around the L & N conductors. You can then check the leakage of each appliance quickly. Very handy.
 18 January 2014 03:28 PM
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broadgage

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Whilst the voltage would indeed probably be higher at night, I doubt that it would increase enough to trip an RCD that does not trip during the day.

Although the insulation resistance readings are worryingly low, they are nothing like low enough to trip an RCD.

If the tripping is time dependant, that suggests a fault in an appliance that is used at night. First suspects IME are appliances innvolving water. Dishwasher, clothes washer.

Another possibility would be a neutral/earth fault in an appliance that normaly pases say 15ma, not enough to trip the RCD.
If some other large load, such as night storage heaters comes on at night, then that can increase the voltage between earth and neutral, and the fault then passes enough current to trip the RCD.
IET » Wiring and the regulations » RCD problem

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