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

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.

The point was to highlight how various small 'adjustments' can put the system on track towards latent failures because 'we' shifted from a direct inspect to an indirect monitoring displayed via an indifferent [to safety] system that can be tweaked by others who have other priorities to the point where our "fail-safe" system has simply been reduced to "Fail".

Once the monitoring is on page 2 of the ''Other Systems" page then a lot of the defence in depth has been lost (the Heathrow transformer 'incident' shows similar 'best laid plans' problems..)

Dear all we seem to have gone off topic; The question -

Is there a reason fuse switches are proposed within the main LV switchboard for sprinkler pump supplies, when all other services in the LV SWB use MCCBs, is the specification of a F/S due to a BSEN or IEC standard, if not, why can I not use an MCCB instead of a Fuse switch? 

I think the topic drifted because you introduced the possibility of monitoring a MCCB using equipment which may not have sufficient integrity to perform that monitoring function.

For me, a good starting point would be: "Does the MCCB manufacturer have a view on their use in circumstances where mal-operation leading to interruption of supply may result in loss of life?".

Fuse reliability (assuming that you buy them from a reputable source) is generally just whatever it is and the designer can pluck a figure out of a standard.  The reliability of more complex assemblies is harder to determine and published figures can sometimes be wide of the mark.

Do you have the necessary reliability data for fuses / MCCBs to be able to demonstrate that using a MCCB was a sensible decision, especially if fuses have been used traditionally?

What happens if the 230v AC nominal or between 216.2V and 253V becomes 207v AC as being proposed? 

 - Will the cartridge fuse or MCCB (Moulded Case Circuit Breaker) perform as expected for time?

 - If you use a device designed for 230V on a 207V supply the device might draw more current to compensate for the lower voltage, which could cause the fuse to blow even during what should be normal operation?

In response to the question "is the specification of a F/S due to a BSEN or IEC standard" - The answer is yes it is likely due to a standard.  Which standard may apply would depend.  As an e.g. refer to BS 8519.  LPC Rules also include a requirement and may also apply .