Three phase heater fusing

I'm not having luck finding a definitive answer to whether I should use Time Delay or Fast Blow fusing for 480V, three phase, 36kW heaters. I've always used time delay (Littelfuse Type 'J'), but was called on this. Anyone have experience or a reference source that might help me?

RBergmann said:

I'm not having luck finding a definitive answer to whether I should use Time Delay or Fast Blow fusing for 480V, three phase, 36kW heaters. I've always used time delay (Littelfuse Type 'J'), but was called on this. Anyone have experience or a reference source that might help me?

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Definitive answer - fast-blow. The only reason to use a time-delay is to allow a motor to have in-rush current for starting.

This is one of those - there is no wrong answer questions ... since a time-delay won't 'hurt'. It just takes a few seconds longer to trip the fuse. If the power leads arc to ground, that amount of time won't make a big difference. If the leads short together, the breaker should trip on instantaneous protection.

The boring explanation part:

The heater does not have in-rush current (at least, none of the heaters that I have dealt with). The resistance gets lower the more it heats up, so it starts out low current and rises over a few seconds.

On the off chance that you have PTC (Positive temperature coefficient) heater, which I have not dealt with ... you may have to test it.

Thanks, ‘thingstodo’. Some of what I’ve read, implies that cold medium (these are immersion heaters) can cause inrush above the calculated FLA of the heaters. This is where I’m having a bit of confusion. Designing for heating is different than motion devices.

RBergmann said:

Thanks, ‘thingstodo’. Some of what I’ve read, implies that cold medium (these are immersion heaters) can cause inrush above the calculated FLA of the heaters. This is where I’m having a bit of confusion. Designing for heating is different than motion devices.

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The immersion heaters may go over FLA, but it won't be 10X over like an induction motor. I would not call it inrush though. inrush is typically measured in # of AC cycles. Somewhere around 10 cycles. For an immersion heater you would need to have the high current for minutes. I don't think a time delay fuse will help you.

Heat tape is a good example of what you are describing. At 40F you can run 10 amps and the max length is 200 feet. When everything is shut down in winter, and it's now -40F ... the heat tape takes 50 amps instead. For those rare occasions we have an electrician and a supervisor install a bypass to the fuse, and monitor the current with a clamp-on meter until the system heats up. The bypass is then removed. The paperwork takes almost a day to run through the various approvals. The work takes 10 minutes. And the monitoring is a couple of hours.

How often are you looking at heating up the immersion heater with 'super cold' temperatures? If it is often enough, it may be worth the effort to install larger wiring, breaker, etc and have 2 different sets of protection - 1 for normal and 1 for cold?

On most used heather resistance alloys like Nichrome or FeCrAl (Kanthal) the resistance increases slightly with temperature, but not enough to create a noticiable inrush current on cold start, probably the difference cold to hot current will no be greater than 10%

One thing to take in account with heathers is that they work always at 100% power and they trend to heat up both the fuses and the fuses holder more than for example a motor.
My recommendation is to use fuse no less than 150% nominal current and an oversized fuse base.

I usually talk to a Bussman application engineer when I am not sure what type of fuse to use. I have found these guys to be really sharp on proper fuse selection..(Or the Application engineer from the equipment supplier) Whatever you do don't use "Economy" fuses. That brand often does not blow as soon as it should.

Fast blow fuses for heaters for sure. I usually size them 15-20% higher than FLA, and never had an issue.

Fusing will depend on what you are using for a power switching device.



If you are using solid state relay / power controller you need fast blow fusing. Otherwise the ssr will protect the fuse



If you are using a contactor you could get away with time delay fuses. Fast blow would also be acceptable.

Thanks to all ... I've changed my specifications and purchase orders to Fast Blow.

heaters do not need current safeties other the short circuit, to protect the cable and equipment.
so no need to use fast ones, just protect the equipment, so the value is by the cable and not due to the heater.
if there are electronic devices you will need fast ones to protect the electronics.