Split supply issue

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Dairyspark

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Hi guys,

So, customer rang me about RCD tripping which sounds simple however, set up is 2 shops, ones a hair dresser and the others now a cafe. One DNO supply in and splits to feed the 2 shops. Now hairdressers was rewired recently (not by me) and worked fine allegedly. Cafe crowd have moved into the empty shop next door and now the hairdresser’s RCD is tripping intermittently. Could something on the cafe side cause the hairdressers to trip?

 
TBH you’ve not got enough information to go on.

the recent rewire isn’t any guarantee of a decent installation

is the hairdressers CU a dual RCD or RCBO board?

id be asking the hairdresser to keep a log of the tripping first, then do a site visit to check out the RCD that’s tripping properly

need more info really 

 
It was the DNO that split it, again I assume it had been done previous to any of the rewire work. 
 

Hairdressers is a dual RCD board and the 2nd RCD keeps tripping, and yes the recent rewire means nothing as I have no idea who’s done it and the person that phoned likes to dabble before they phone 🤦🏻‍♂️
 

The sort of main question is can something on the supply side of an RCD cause it to trip? 

 
The sort of main question is can something on the supply side of an RCD cause it to trip? 


Yes, there are certain instances where this can happen, two sceneros spring to mind, but I'm sure there are more:

The first would be if there is a high impedance neutral earth fault downstream of the RCD thats tripping but not normally passing enough current to cause a trip, depending on exact set up, loading in the adjacent unit might drive the voltage between neutral and earth apart and result in more current flowing through the fault.

* This is going to be more prevelent where there is a SNE service cable comming into the building, you have the volt drop on the entire length of N of the servcie cable appearing between N and E, but you could have a much smaller version of the effect over a metre or so of shared N tail even if it comes in as CNE, basically its voltage drop on the neutral from the point it which it stops being one with the earth.

The second, would be, I've seen a poor connection before an RCD, trip the RCD, basically I think when a certain amount of load is applied, partically if its not a resistive one. The poor connection is arcing and there are all sorts of frequencies and noise appearing on the supply, this either confuses the electronics in the RCD, or actually causes earth leakage trhough the inherrant capactance of the cables/loads when exposed to higher frequencies than they are expecting. It was a house with a main RCD board, the RCD would be fine most of the time, but when the vaccuum cleaner was used it would nuisence trip often. The poor connection was in the neutral henley block before the DB and could be heard fizzing when the vaccumm was in use. Fixing this cleared up the nuisence tripping issue.

 
I would be inclined to look closely at the primary bonding, especially if there is any shared plumbing between to two units.

What makes you think its the cafe system's fault though?  Likely a dodgy kettle or similar in the hairdresser's.  Does the cafe have RCDs?

 
I agree I would rule out an issue with the hairdressers, the tripping along with cafè opening could be pure coincidence. 

 
Last edited by a moderator:
apart from the above comments, I suspect the RCD tripping is merely an appliance fault. At a guess, the hairdresser never used to have any RCDs, and so aren't used to dealing with RCDs tripping. As a commercial building, are RCDs really needed except for the sockets? I would also be looking at changing to full set of RCBOs to improve reliability. 

 
apart from the above comments, I suspect the RCD tripping is merely an appliance fault. At a guess, the hairdresser never used to have any RCDs, and so aren't used to dealing with RCDs tripping. As a commercial building, are RCDs really needed except for the sockets? I would also be looking at changing to full set of RCBOs to improve reliability. 
agree 100%

why do people fit dual rcd boards in commercial properties? 
 

RCBO boards with mcb for circuits which don’t need RCD protection.

 
apart from the above comments, I suspect the RCD tripping is merely an appliance fault. At a guess, the hairdresser never used to have any RCDs, and so aren't used to dealing with RCDs tripping. As a commercial building, are RCDs really needed except for the sockets? I would also be looking at changing to full set of RCBOs to improve reliability. 
Depending on installation methods additional rcd protection may well be required for anything outside of socket outlet circuits. 

 
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