• Welcome to the new forum. We're now hosted on ElectriciansForums.net rather than ElectriciansForums.co.uk - Chat about the forum upgrade by CLICKING HERE

Welcome to our Electrical Forum - We started out providing electrical advice just for the UK - But now we have specific advice for the USA too! - Register for free and share American Electrical Advice.

Discuss Brainteaser in the Commercial Electrical Talk area at ElectriciansForums.net

Status
Not open for further replies.

shugs11

EF Member
Working on a large commercial job, each floor has 2 lighting and 2 power boards and presumably others for mechanical etc. One of the RCBOs was noticed to be overheating/melting and was assumed faulty so changed. After a few days, melting again, so it was changed again. The next time it happened some clever chap thought it might be a good idea to investigate why.
What he found was 2 lighting circuits both on the same phase feed from different boards on the same floor, someone had accidentally put a link between between the 2 feeds.
One circuit was fed via a " C10 Type A single pole RCBO " ( which overheated )
The other circuit was fed via a " C10 Mcb " which was seemingly unaffected
When I quized him he couldn't be certain whether or not there was any load on either circuit but he had checked between the 2 boards on the phase concerned and there was no PD


I'm curious as to what exactly happened. Does anyone have any ideas ?
 
W

WPD Lad

You've in essence got 2 sources in parallel with differing impedances this creates circulating current from one source to the other, I'm not sure it'd melt an RCBO though.

We have to watch the circulating current when we parallel two primary substations as it can soon get out of hand and blow stuff up
 
W

WPD Lad

When you have differing lengths of cables with different loads, joints, CSA etc then you'll get a difference in impedance between the two circuits.

Imagine it as 2 radials joined at the ends, if one radial is longer and has more load then the volt drop at the end of the run will be greater than the volt drop at the end of the other run. This creates a potential difference (voltage) and a current to flow from one source to the other.
 

shugs11

EF Member
Thanks that's something along the lines of what i was thinking when i was told about it but i can't see how it would have melted the RCBO.
Is it possible that the RCBO was in the off position, it being single pole the current was still able to flow in the neutral and caused it to overheat ?
 
K

Knobhead

The RCBO and MCB along with the connecting cable will be trying the balance the loads between the two boards. This can have catastrophic effects. As has been said an imbalance in voltage due to the board supply impedance will cause a current to flow to equalise the voltage difference.
Please check the neutrals aren’t cross connected. As the neutral has no OCP the danger is even worse.
 

shugs11

EF Member
Apparently the neutrals were checked first and found to be fine. After the problem with the linked feeds was discovered that too was made right. Surely if there was no PD between the two boards after the link had been removed then there would have been no PD before the link was removed wouldn't this mean there was no imbalance ?
 
W

WPD Lad

No pd between the boards at all?

The largest pd would be between the ends of the radials
 
Last edited by a moderator:

shugs11

EF Member
That's true but it won't be feesible to check that now, besides wouldn't this only be a problem if there was a load on either of the two circuits ?
 
K

Knobhead

The best way I can describe this.
View attachment 10842
What you found yourself with was two boards with a voltage difference between them. OK it may only be a matter of a couple of volts, but they will try to balance them selves out. This gives you the circulating currents. I’ve seen 185mm cables fried due to this.
 
Status
Not open for further replies.

Electrician Talk

Electrical 2 Go - Electrical Supplier and Electrical Wholesalers
Electrical Insurance
ElectriciansForums.net (formerly known as ElectriciansForums.co.uk) was setup in April 2006 to provide free electrical advice to both professional and DIY electricians. Whether seeking advice for auto electrics, trying to find electrical installation condition reports (EICR) form downloads, or whether you're here just to talk to other electricians; we've been a first-choice website for tens of thousands of sparkies for well over a decade.
Top