Unusual RCD problem

[jibjob]

Here's the senario. Regency building with 5 flats contained within. System is 3 Phase with a further 2 service fuses added on making 5 in total, earthing is TT. Each flat is fed via a mixture of DP 100mA or 30mA RCD's which in turn feed KMF switched fuse isolators. 16mm T&E tails to each flat (old skool), most of which have Wylex boards with BS EN 3036 Semi enclosed fuses.

Tenant on ground floor reported fault with gas cooker tripping RCD when he used either the oven light or the ignition. When his RCD tripped it similtaneously tripped the RCD feeding the basement flat. Apparently this has happened on a number of occasions but was never reported.

Ground floor flat is 100mA RCD, basement 30mA. Tested both RCDs, 30mA tripped within specified times and at 27mA when ramp tested. The 100mA however would trip within specified time at x1 on 0 & 180 degrees (without tripping the other RCD) but my Megger gave me the message >50V when attempting x5 test. Zs at socket outlet is 118 ohms.

Simulated similtaneous RCD trip by plugging in cooker & switching on oven light.

I'm kinda thinking that there may be a borrowed neutral (or an entire circuit!) between both flats but have never encountered this type of fault before. It was late in the day so suffice to say I will be going back in next few days to try & sort it.

Anybody else come across this before? Any help greatly appreciated.

 

[MarkieSparkie]

... Ground floor flat is 100mA RCD, basement 30mA. Tested both RCDs, 30mA tripped within specified times and at 27mA when ramp tested. The 100mA however would trip within specified time at x1 on 0 & 180 degrees (without tripping the other RCD) but my Megger gave me the message >50V when attempting x5 test. Zs at socket outlet is 118 ohms. ...

x5 Test is only required for 30mA RCDs providing additional protection, so the result or lack of it is not important.
Generally the MFT or RCD tester will time out on the x5 RCD tests when applied to RCDs with higher nominal current ratings, rather than indicating over touch voltage limit and inhibiting the test, but check the operating manual for your MFT or tester.
If you are testing an S type time delayed RCD you may need to select a special mode on your MFT or RCD tester for accurate results on this test.
Also, an over touch voltage limit warning and inhibited test can often due a poor connection on one or more of the test leads, usually E and/or N, and not a real high touch voltage. Particularly likely where croc-clip connections are used.

 

[SirKit Breaker]

Sounds like the cooker is goosed to me. Not uncommon. Can you get hold of a PAT tester that measures earth leakage, test it and see what the results are like.

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

 

[spark 68]

but my Megger gave me the message >50V when attempting x5 test. Zs at socket outlet is 118 ohms

You don't test 100mA RCD's @ X5, only 1/2 and X1, 0 and 180 degrees, so not suprised you got >50V touch!

The only RCD's to be tested @ X5 Idn are 30mA ones or less, used for additional protection.

Edit:Sorry, Mis-read Op

 

[jibjob]

For sure the cooker has a fault, what I'm trying to establish is why would the RCD serving the ground floor flat & the RCD serving the basement trip similtaneously when the cooker is plugged in. Both flats are independant of each other, the RCD's are both situated in the meter cupboard in communal hallway. I'm going back next week but would appreciate any thoughts you guys might have

Cheers

 

[spark 68]

Is the 100mA RCD an 'S type' (time delayed) ?

 

[jibjob]

You don't test 100mA RCD's @ X5, only 1/2 and X1, 0 and 180 degrees, so not suprised you got >50V touch!

The only RCD's to be tested @ X5 Idn are 30mA ones or less, used for additional protection.

Edit: Mis-read Op

That'll be why then

 

[jedisparks]

Had a similar problem to this recently. A big house with an annexe. House on one RCD, the annexe on another. Thought to be completely seperate wiring. A fault was causing both RCDs to trip similtaneously. Turned out to be a faulty outside light on the house side. In addition some wise guy borrowed a neutral from the upstair lighting circuit in the house to make an extractor fan work in the annexe. took a while to find.

This could be a similar thing.