Thermal vs Magnetic Circuit breakers (Float switch)

[Scubadoo]

Hi All, Looking to replace a faulty shower pump float switch with a Whale Bilge Water level sensor (no moving parts etc), but reading the instructions it clearly states "Do not use thermal circuit breakers with the BE9003(B)/BE9006(B). Magnetic breakers must be used".

Can anyone explain why? my boat and I think probably many others have Thermal circuit breakers, so it looks like a non starter for me as my existing float switch is connected a Thermal breaker. It also seems magnetic breaker aren't so readily available like the Thermal units.

This is the unit: Whale Bilge Water Level Sensor BE9003

Thanks.

 

[andsarkit]

Magnetic breakers will trip immediately the current is exceeded. Thermal breakers will have a delay while they heat up and will also vary their sensitivity with ambient temperature. This delay is often useful to allow a current surge when the motor starts up.
Often the two are combined to get the best of both characteristics.

I'm not sure why you can't use a thermal breaker - you would have to ask Whale. I can only think that the electronic switch is only rated for sufficient current for normal use and it could be damaged by the surge that could get through a thermal breaker.
RS and CPC Farnell sell a variety of circuit breakers from suppliers such as ETA and Carlingswitch. You might find one that matches your existing switches.

 

[superheat6k]

If this were me I would use an A or B curve breaker. That said a bilge pump is still a motor so possibly too much surge for an A rated MCB, but I doubt most bilge pumps will require 20Amps. So if your pump requires say 10Amps then I can't see why a 12 - 16 Amp MCB would not work adequately.

 

[trbt]

In a fault situation the electronic switch will protect the thermal breaker by being quicker to blow up.

 

[Scubadoo]

Magnetic breakers will trip immediately the current is exceeded. Thermal breakers will have a delay while they heat up and will also vary their sensitivity with ambient temperature. This delay is often useful to allow a current surge when the motor starts up.
Often the two are combined to get the best of both characteristics.
View attachment 142364
I'm not sure why you can't use a thermal breaker - you would have to ask Whale. I can only think that the electronic switch is only rated for sufficient current for normal use and it could be damaged by the surge that could get through a thermal breaker.
RS and CPC Farnell sell a variety of circuit breakers from suppliers such as ETA and Carlingswitch. You might find one that matches your existing switches.

Thank you for taking the time to explain, very helpful. I am reluctant to replace my existing breaker and the added hassle as I was simply thinking of swapping over the float switch. Thanks for the vendor references, will have a look.

I think my other option would be to add an inline fuse at the float switch.

 

[Scubadoo]

If this were me I would use an A or B curve breaker. That said a bilge pump is still a motor so possibly too much surge for an A rated MCB, but I doubt most bilge pumps will require 20Amps. So if your pump requires say 10Amps then I can't see why a 12 - 16 Amp MCB would not work adequately.

Thanks, the bilge pump is low amperage, max 4amp.

 

[andsarkit]

I think my other option would be to add an inline fuse at the float switch.

Fuses also come with different characteristics like the A, B, C, D ratings of MCBs. If the amp rating is followed by an F it is a fast fuse. If followed by a T it is a time delay anti-surge fuse.
If your pump is only 4A and you have a 5A or 6A thermal trip I'm sure that will be fine as the current will trip well before 20A.