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Multi-output relay protection

vantech

Hi, 

I am looking at using an electronic relay with 1 single phase input and 4 switched outputs.

Each output can be loaded to a max. 16A with a device maximum of 40A. This means the relay itself (its input) would need to be protected by a 40A breaker. The input is split between 4 outputs internally. 

The question I have is regarding the outputs. Obviously 16A on a 2.5mm cable is fine in most circumstances, however would a 40A MCB suffice for short circuit protection, if the loads connected to the relay are known (e.g. electrical heaters, pulling say 13A). The fixed load would ensure protection against overload, but would the above provide adequate short circuit protection? 

Would I be better installing additional MCBs on the relay outputs, even if all loads are known/fixed? 

Thanks. 

Parents
  • 0

    2.5mm² is probably fine on a 40A MCB (provided it's not a 10kA version) for faults as the generic max energy let-through should be under 66,000 A²s. You'd still need to make sure that the loop impedances where low enough to get instantaneous operation of the 40A device though - which may or may not be a challenge, depending on the exact device, cable lengths and supply involved.

    I've not come across a beast with one input and 4 outputs - do you have a particular device in mind? I would have thought a more usual arrangement would be either 4off single pole devices or a 4-pole device with separate input terminals - that way you could have your 4 16A MCBs upstream of the relay and ensure full protection throughout (including for the relay itself, which may require some study of section 536 and data from the contactor (relay) manufacturer).

       - Andy.

Reply
  • 0

    2.5mm² is probably fine on a 40A MCB (provided it's not a 10kA version) for faults as the generic max energy let-through should be under 66,000 A²s. You'd still need to make sure that the loop impedances where low enough to get instantaneous operation of the 40A device though - which may or may not be a challenge, depending on the exact device, cable lengths and supply involved.

    I've not come across a beast with one input and 4 outputs - do you have a particular device in mind? I would have thought a more usual arrangement would be either 4off single pole devices or a 4-pole device with separate input terminals - that way you could have your 4 16A MCBs upstream of the relay and ensure full protection throughout (including for the relay itself, which may require some study of section 536 and data from the contactor (relay) manufacturer).

       - Andy.

Children
  • 0 in reply to AJJewsbury

    Thanks Andy.

    The relay itself has an input, which is then essentially split between 4 relays. The relays then have their own individual outputs.

    I think I’ll go for a breaker after each output, just to be on the safe side. 

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