Protective device for pump motors on a fire sprinkler systems

Fuses are often a requirement of the fire system design. 

Fuses are very, very, reliable.

MCBs are only very reliable - and in principle an MCB might have had some full current trip event in the past, and be in some way weakened, and you cannot tell from outside, or indeed it could just get knocked off and not be spotted.

Generally the idea here is that the pumps must be certain to keep turning right to the last possible moment. -  if required that includes while the water runs out or the burning building pretty much falls down around them.

How important that is also depends if there are redundant pumps or redundant supplies to the pumps.

Mike.


PS DIN rail fuses are a common neat and tidy compromise.

Hi Mike, thanks for the response. To confirm, the F/S units are built into the floor‑standing 3200A LV switchboard located in the switch room.

Regarding your comment that fuses are “very, very reliable,” fuse cartridges can still degrade over time due to motor load demands during annual testing. There’s no way to verify whether a cartridge will perform correctly under high‑demand conditions, and a locked‑rotor event may lead to only the affected phase being replaced. Such events cause pitting and heat buildup on contact surfaces, accelerating deterioration. A fuse switch offers no means to check this, and a single thermal image can be misleading.

Both fuse switches and MCCBs have rotary handles that can be locked in the closed position, with red‑painted covers for identification. Since only authorised personnel access the LV sub, the risk of accidental operation applies to both devices.

MCCBs, however, include wear indicators that allow monitoring of both the device and the trip unit via the BMS. Trip units can be set to match or exceed fuse curves to account for cartridge tolerance. Provided the building’s cabling and support systems meet the standard, pump supplies will continue operating until cable damage prevents it.

Given all the above, is there a particular reason fuse switches are used within the LV switchboard for sprinkler pump supplies when all other services use MCCBs, is the F/S requirement due to a BSEN or IEC standard, or just because this is the way it has been done before? 

Philip

Good questions, and not ones I can really answer - there are others who frequent the wiring forum who do this sort of thing regularly for a living, I suggest we wait for one or two to chip in.

However it may well be as you suggest  that improved equipment standards mean there are very good reasons to re-evaluate alternatives to the traditional approach. I suspect part of the reason to move slowly however is that the whole point of such systems is that ideally they get used very rarely, indeed hopefully never except during inspection and test, so there won't be a lot of data on failure modes in actual use to show what really happens in practice.

Mike.

Philip Baker said:

wear indicators that allow monitoring of both the device and the trip unit via the BMS

Is the BMS software written to safety critical standards? Just saying. If it's not then such monitoring isn't as "safe" as you may hope.

See recent discussion  RE: From Digital Technology to Rail Systems: Reflections on Building Reliable, Safe Infrastructure  

Philip

Philip Baker said:

wear indicators that allow monitoring of both the device and the trip unit via the BMS

Is the BMS software written to safety critical standards? Just saying. If it's not then such monitoring isn't as "safe" as you may hope.

See recent discussion  RE: From Digital Technology to Rail Systems: Reflections on Building Reliable, Safe Infrastructure  

Philip

Is the BMS software written to safety critical standards?

And perhaps not all safety critical standards are equal in that respect. In "normal" situations you might expect a failing protective device to "fail safe" by opening the circuit and so removing the risk of shock or starting fires, but with the supplies to fire fighting equipment safety is often best served by the keeping the equipment on-line for as long as possible - i.e. if anything the protective device should fail safe by keeping the circuit closed, not open.

I must say there's something reassuring about metal conductors bolted to the tags of a ceramic fuse cartridge - even if everything around it melts away or gets coated in carbon deposits, it's likely to keep going - perhaps less so with circuit breakers where contact perhaps depends more on spring pressure and it all being held together by plastic cases, let alone anything with electronics in it.

   - Andy.