Discuss What would give a low N-E resistance (at DC), but not cause the RCD to trip? in the Electrical Forum area at ElectriciansForums.net

happysteve

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I'm familiar with RCD spurs giving readings of 0.1 to 0.3MΩ when testing insulation resistance (L+N tied together to E), but what would give a reading of about 65Ω (when tested with "low resistance ohmmeter" i.e. continuity setting) N-E? On a lighting circuit - N isolated from other N's protected by the same RCD, and just for good measure the (double pole) RCD turned off (so if there's a borrowed N with another circuit, N on the circuit is still isolated from E at the PME block). RCD tests out fine, no reports of tripping.

I'll admit I didn't clamp L+N to see if there was actually any leakage... my working assumption is that this is an AC/DC thing - in the same way that if you measure the resistance across L and N when there's a transformer in the circuit (e.g. a shaver socket) it gives a very low reading, but it draws nothing at 50Hz.

I couldn't see any RCD spurs, but that doesn't mean there isn't one buried away somewhere... any I've never had a strange reading this low before.

What could it be?!?

Cheers :)
 

littlespark

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Is the rcd a true double pole? Disconnect the circuit and test again.

Could be any number of things, likely squashed or nicked cable somewhere.
 

Wilko

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Hi - my quick thought is - if the DC reading was 65 Ohms but the AC impedance at 50Hz was much higher this implies an inductor (a choke).
 

Lucien Nunes

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65 ohms is likely to be a fault where N and E are just barely in contact with one another. This might not trip the RCD if it's near the DB, especially on PME, because as radiohead points out there might never be enough voltage drop along the N conductor from DB to fault to drive 20+ mA through it.

I think the inductance idea may be a red herring. There's no reason for anything inductive to be connected between N & CPC, unless it's actually connected between L & N and the L is disconnected, etc, a bit far fetched.
 

happysteve

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  • Thread Starter Thread Starter
  • #6
Thanks all. Appreciate the responses. :)

In reply to some of the above:

- yep, definitely a double-pole RCD

- yep, aware of the usual causes of N-E shorts/low resistances

- thought about something inductive, but couldn't think of anything. As I said, I'm aware though of some items (like some BS7288 RCDs) that do give a low reading between the live conductors and the cpc, but usually in the 100's of kΩ, rather than 10's of Ω

- hadn't worked through the maths with respect to cable resistance vs fault resistance vs load current --> leakage current, I will have a think :)
 

pc1966

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At 65 ohms you would need 1.95V between N & E at the origin to cause 30mA, so if the voltage is generally below 0.975V (which is likely in a TN-C-S setup) then you won't see enough earth current to cause a trip.

However, in many cases what you will see if it tripping unexpected when you have, say cooker on and shower in use, or vacuum cleaner started, and the peak demand goes to 60A or so then you would see that sort of voltage if there was a total of 16.25 mOhm resistance between the neutral bar in the CU and the combining point of the PME N+E.

That sort of resistance is rather high for that part of the installation, so unless you have very long tails (17m of 25mm, for example) and/or some dodgy contact resistance, it is not going to show up routinely.
 
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