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# Brainteaser

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

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#### 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

Circulating current?

#### shugs11

##### EF Member
can you explain that for me please i'm not quite sure what you mean

W

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

#### shugs11

##### EF Member
i'm not sure i follow what you're saying about the differing impedances.

W

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

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.

W

#### 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

No pd between the boards at all?

The largest pd would be between the ends of the radials

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#### 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