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Topic Title: RCD problem
Topic Summary: Ohms law
Created On: 17 January 2014 08:37 PM
Status: Post and Reply
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 18 January 2014 06:26 PM
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WiredScience

Posts: 116
Joined: 25 January 2012

+1 for an earth leakage clamp meter. I recently clamped a single RCD+15MCB installation which was randomly tripping.

IR of whole board lives to earth > 120MOhms. Measured leakage 21 mA with all the PCs, TVs, games and gadgets on. Supply voltage at time of measurement was 234V.

The (unloaded) RCD consistently tripped at 22mA when ramp tested.

Assuming a linear relationship between supply voltage and leakage current, then a 5% increase may cause a trip.
 18 January 2014 07:37 PM
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Jaymack

Posts: 4785
Joined: 07 April 2004

Originally posted by: WiredScience
Assuming a linear relationship between supply voltage and leakage current, then a 5% increase may cause a trip.

Any increase at all in voltage or decrease in the resistance, will cause a trip; if the circuit was already on the threshold of tripping the RCD.

Regards
 18 January 2014 07:55 PM
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Phillron

Posts: 1261
Joined: 18 January 2007

There is a pecking order for suspicion when intermittent and witching hours tripping occurs

The top of the list is appliance fault
The top of that list is the Fridge freezer
The suggestion of using a clamp meter,unfortunately it needs a lot of patience to stand there watching and waiting until that freezer stat decides to let the whole of its function become part of your game

Your meters have not revealed your problem and likely will not do so

Run that appliance from a different source for a few nights and see what developes
 19 January 2014 07:44 PM
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Jaymack

Posts: 4785
Joined: 07 April 2004

Originally posted by: Phillron
There is a pecking order for suspicion when intermittent and witching hours tripping occurs

In my personal experiences for tripping RCD's at any time, are heating grills, electric irons, electric ovens, tumble dryers and toasters are top contenders. (All with heating elements).

Regards
 20 January 2014 10:17 PM
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AJJewsbury

Posts: 11768
Joined: 13 August 2003

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

Yes (L-earth, presuming 240V).
- Andy.
 21 January 2014 12:16 PM
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WiredScience

Posts: 116
Joined: 25 January 2012

Originally posted by: AJJewsbury

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


Yes (L-earth, presuming 240V).

- Andy.


Not forgetting that the d.c. tester won't measure the true impedance at 50Hz, so there may well be some disparity between calculated and actual leakage current.
 31 January 2014 08:15 PM
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Grumpy

Posts: 460
Joined: 09 January 2009

Sorry to resurrect an ageing post. Was called back to a job where I had replaced a pendant with a fluorescent. Three months later, when it was switched on it tripped the RCD. It was loop in to switch so easy to test switch to lamp cable, IR 0.2Mohm. Ohms law gives 1.15 mA. So why would that trip a 30mA RCD? Je pose ma valise from my first response.
 03 February 2014 03:32 PM
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AJJewsbury

Posts: 11768
Joined: 13 August 2003

It was loop in to switch so easy to test switch to lamp cable, IR 0.2Mohm. Ohms law gives 1.15 mA. So why would that trip a 30mA RCD?

Just one guess - during startup the fluorescent generated a higher 'striking' voltage - if that was across the fault a lot more that 1.15mA might flow?
- Andy.
 03 February 2014 04:30 PM
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webbix

Posts: 134
Joined: 29 October 2006

Many thanks for all your advice. after having tested the ring upon reconnecting the washing machine with the machine switched off it tripped . then after resetting RCD i plugged the unplugged machine several times on about the 5 attempt it tripped again.

it failed the PAT test earth leakage 0.26mA but trips after several attempts DEFO the machine . But why it trips in middle of the night when not switched on (but plugged in) i dont know!!
john
IET » Wiring and the regulations » RCD problem

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