Cellar cooling fuse rating

[IAmSparkytus!]

A cellar cooling system with the main supply going to the indoor unit.

Spec sheet gives the following figures...

12.1amps FLA
65amps LRA
32amp fuse recommended

My question:

Why a 32amp fuse? Have they based this on HRC fuses perhaps and not advised that?

Wouldnt a C-curve 16amp 60898 be sufficient based on those FLA and LRA numbers?

Thanks.

 

[pc1966]

This has some useful info:

https://www.hager.co.uk/files/download/0/104407_1/0/TRANSFORMERPROTECTIONVALUEV2.PDF

Based on the figures you have it looks like a ~2hp DOL single phase motor, so 32A C-curve MCB would appear to be a reasonable alternative to an HRC fuse (and of course could be RCBO).

 

[IAmSparkytus!]

This has some useful info:

https://www.hager.co.uk/files/download/0/104407_1/0/TRANSFORMERPROTECTIONVALUEV2.PDF

Based on the figures you have it looks like a ~2hp DOL single phase motor, so 32A C-curve MCB would appear to be a reasonable alternative to an HRC fuse (and of course could be RCBO).

Thanks for the info.

I figured a C32 would be too big and a C16 would be better considering the normal running current of 12.1A and a C16 would cope with the 65A start-up current due to the 5-10x tripping characteristics

 

[pc1966]

I figured a C32 would be too big and a C16 would be better considering the normal running current of 12.1A and a C16 would cope with the 65A start-up current due to the 5-10x tripping characteristics

I think in the more general motor start case the assumption is the run-up can be slow so might cross the corner of the thermal overload part.

Usually though they would give recommended MCB as well as fuse ratings to work from though.

 

[IAmSparkytus!]

So in this case, the manufacturer has recommend "fuse rating 32A". Would you personally still fit a C32 as they've recommended 32amps or would you ever be tempted to downgrade the fuse rating knowing a C-curve will stop tripping on start?

 

[pc1966]

So in this case, the manufacturer has recommend "fuse rating 32A". Would you personally still fit a C32 as they've recommended 32amps or would you ever be tempted to downgrade the fuse rating knowing a C-curve will stop tripping on start?

Here you are only needing to supply fault protection, so if 32A C is meeting Zs and cables are safe to 32A then not much point in worrying about it.

Generally HRC fuses have a better combination of energy-limiting and max Zs for a given running current and I quite like them but I'm also aware are really not a great idea for any system that lacks skilled supervision (or needs to cut all phases on single phase fault), so MCB/MCCB can be better overall even if technically inferior on the protection aspect alone.

If I was doing more of the product design then I might look at what the motor starter/overload arrangement was and if it is better to look at an integrated motor control/protection breaker, these sort of things:

SM1R1400 | Lovato 9 → 14 A Motor Protection Circuit Breaker | RS

uk.rs-online.com

MPW40-3-U010 | WEG 6.3 → 10 A Motor Protection Circuit Breaker | RS

uk.rs-online.com

 

[IAmSparkytus!]

I was thinking more for overload protection. If the motor is only supposed to be running at 12.3A, surely letting it continuously run at say 25-30A as a 32A breaker would can't be good for it?

 

[pc1966]

I was thinking more for overload protection. If the motor is only supposed to be running at 12.3A, surely letting it continuously run at say 25-30A as a 32A breaker would can't be good for it?

Is there an overload relay as part of the fan control? E.g. contactor block has one built in.

If not then yes, it would be an idea to look at some better motor protection and then usually the overload relays can have the expected running current dialled-in so you get tailored protection.

A normal MCB or fuse is not very good for overload protection here, but fine for fault protection (sometimes also going if motor locked and not just slowed down). Hence the modern trend for "motor protection" breakers to reduce the number of separate parts to be wired up to achieve it.

 

[IAmSparkytus!]

Is there an overload relay as part of the fan control? E.g. contactor block has one built in.

If not then yes, it would be an idea to look at some better motor protection and then usually the overload relays can have the expected running current dialled-in so you get tailored protection.

A normal MCB or fuse is not very good for overload protection here, but fine for fault protection (sometimes also going if motor locked and not just slowed down). Hence the modern trend for "motor protection" breakers to reduce the number of separate parts to be wired up to achieve it.

I'll have to check regarding the overload relay but thinking about it I'd imagine there is, yes.

I'd still have thought the smaller the fuse size, the safer it would be though. Obviously ensuring its not too small that it would be tripping for no reason.