Split Load Consumer Unit Question.

[Paul Newmarket]

Hi. Can anyone explain the following?

You have a 100A trip on the incoming then two RCDs typically 80A and 63A.

The 80 and 63 are RCDs not RDCOs, therefore trip on earth leakage (30mA?) but not on overcurrent.

What happens if one half is pulling 90A? The RCD is over stressed but the 100A trip doesn't cut out?

Perhaps I've got it wrong and the 63A and 80A are overcurrent as well as RCD.

 

[yellowvanman]

Typically on a dual RCD board there is no 100A trip - its a simple DP isolator - unless its an older TT system when it could be a 100mA RCD.

When you say stressed - the current rating of the RCD is its ability to break a certain current in a fault condition. So if 1 of the RCDs has 90A flowing through it, there maybe an issue when its tries to trip, but there shouldn't be a problem with the 90A flowing through it.

Obviously if you could forsee this problem you would need to distribute your circuits intelligently

 

[KFH]

I would plan for the expected currents on the MCBs not to exceed the switching capacity of each RCD to which they are connected. The MCBs provide the overcurrent protection and the RCDs the earth fault protection, there are two so that faults on one do not trip the whole installation. The 100A main switch is usually just a switch, you can have a time delayed RCD in there for TT installations usually 100mA. The time delay is to provide discrimination as normal RCDs in series may not discriminate. BEAMA do a handbook for RCDs, I do not thave the web address but a search should find it.

 

[pushrod]

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What happens if one half is pulling 90A? The RCD is over stressed but the 100A trip doesn't cut out?

.

The total amps of the mcbs after the rcd is very unlikely to be anywhere near the actual current that flows thru the rcd. If however you do get very large currents flowing (maybe shower, oven and sockets all on same side) this in my experience can be a cause of rcds failing. As yellowvanman said you need to redistribute the circuits better to avoid overloading. The main switch (rated to switch a 100A load) is just that and not an mcb. Exceed 100A for a long time and the distributors main cut-out fuse will blow.

 

[Octopus]

Hi. Can anyone explain the following?

You have a 100A trip on the incoming then two RCDs typically 80A and 63A.

The 80 and 63 are RCDs not RDCOs, therefore trip on earth leakage (30mA?) but not on overcurrent.

What happens if one half is pulling 90A? The RCD is over stressed but the 100A trip doesn't cut out?

Perhaps I've got it wrong and the 63A and 80A are overcurrent as well as RCD.

If your example is a house, I very much doubt the total load would get anywhere near 90A let alone 90A on one RCD. Sure the total of the MCB's will often be way over the size of the main fuse and therefore over the RCD's. The key is your judgement to split the load evenly, and also make sure you don't have the ground floor lights and ground floor sockets on the same RCD.

Hope this helps