fuse protection for resistive heater load 1

[robbm]

We use immersion heaters in piping and some customers have experienced issues with the fuses blowing. We typically have 30kW heaters (@460VAC), for a load of under 40A and use 50A high-speed style fuses (Bussmann FWP series). It certainly seems as though it's an operator issue, since we don't have the issue with it in-house. My boss asked me to look into using different fuses or something to prevent this/fix this for the customer in his ear currently. I was told the customer is more concerned with getting running again than the consequences of doing it wrong or just putting in larger fuses, etc. Other than protection of the equipment (which the end user seems unconcerned about), are there any other reasons I can give my boss to avoid having them switch from a high-speed to a time-delay fuse? I hate giving out bandaids, but sometime my hands are tied unless I can give a good reasoning for not using the bandaid...thoughts?
thanks in advance

 

[DRWeig]

Hi robbm,

What's your reason for not wanting to use a dual-element fuse?

Could the customer's fuse blowing be due to extra-cold liquid in which the heater is immersed when started, as opposed to the liquid you use for testing in-house?

Electric heaters have a pretty good inrush when cold. I have always had either a time-delay or dual-element specified. I don't see any restriction in the codes or UL standards that would require an instantaneous-only device.

Good on ya,

Goober Dave

 

[robbm]

i didn't see a UL requirement either, but we've used the high-speed type since before my time here and i've been trying to find the reasoning beyond just protection of the device. we have had the heaters burn up from being on and running without water, etc, even for a short time, so we definitely have reason to use the high-speed. i didn't know if anyone else uses high-speed and could offer another reason or if we should switch to dual-element time delay instead.

Regarding the coldwater/inrush...Aren't electric immersion heaters resistive loads, which means they wouldn't have an inrush like an inductive load?
These are the heaters we use:

http://www.asbheat.com/01_html/products/01_screw_plug_heaters.shtml

thanks for the thoughts...

 

[Compositepro]

The cold resistance of the heating element is significantly lower than the hot resistance, so there is some "in-rush". It's not as great as starting a motor but can last longer.

 

[robbm]

ok, thanks. that makes sense, since UL508A also states the heater loads must be sized for at least 125% the heater load current (so a 50A is about the lowest to the 37A load as we can go). If it isn't a significant inrush, i wouldn't think a 50A fuse should be blowing from just coldwater being used...

 

[robbm]

another option my boss asked was if there's any harm in using a 3pole UL489-rated CB in place of fuses. i'm trying to go through curves of certain breakers vs. the highspeed fuses vs. dualelement time delay. any thoughts?

 

[DRWeig]

Hi again robbm,

Thermal-magnetic breaker is fine too, in my opinion.

Couple items:

1. As compositepro said, there is inrush. It can be quite significant, but of short duration -- hence the blowing of a super-fast fuse. Power up a cold heater with an oscilloscope attached to indicate magnitude of current (voltage drop along one lead, for example). Then try an extra-cold heater. Magnitude of inrush versus steady-state depends on ultimate temperature of the heating element versus its temperature on start-up.

Example: Incandescent light bulb filaments can bring 10X or better inrush when really cold at startup. It's short duration, but it can play heck on switch contacts. I doubt that your heater gets to the temperature of a light bulb filament, but it probably gets pretty warm anyway.

Again, I've always used a dual-element fuse or a thermal-magnetic breaker.

2. Heater left with no running water will not be saved by a fuse or breaker, either one. The heating element will get much hotter and thus increase its resistance. Current will fall, not rise. Heater fails because it just gets too hot and maybe melts. If the ultimately-burned heater leads fall together in this instance, you'll get a fault current to interrupt - but your heater was already toasted.

I'm going to opt to disagree that a fast-blow fuse is necessary, or even desireable, in a resistance heat circuit.

I'd like to hear the others chime in though!

Good on ya again,

Goober Dave

 

[robbm]

dave,
thanks. looks like whomever here started the process of using the highspeed fuses was misguided, so we'd probably be best switching to a thermal mag or a dual element. surprised now to think we haven't had more issues with fuse blows at startup more often...
thanks again.

 

[Compositepro]

The cold water is not the issue. The resistance wire inside the heater sheath maybe at over 1700F when at full power. It is this temperature which sets the limits on the power the heater can produce. Over this range Nichrome 60 resistance increases about 14%. Tungsten light bulb filaments increase resistance by a factor of 15. I don't know the specs for your heater.

 

[robbm]

that helps, thanks. our heaters are only 50-250F heaters, so the resistance should likely be less than the 14% in your example, correct?

 

[zeusfaber]

I bet the resistance wire runs much hotter than this: without a decent temperature differential, you're not going to be able to drive heat through the thermal resistance of the insulating packing. A.

 

[DRWeig]

Compositepro is probably closer to right than I am on the resistance change --

But I'll still hold to my recommendation. No need for a fast-acting protective device for a heater.

Been using same standard thermal-magnetic breakers on electric duct heaters and water heaters in your wattage range for years.

Good luck with your trouble-solving!

Good on ya

Goober Dave

 

[jraef]

"High Speed" fuses are high in terms of milliseconds! Nothing a resistive heating element can do needs that level of protection, and nothing about a high speed fuse is going to make a hill of beans difference in protectiong your heater element (I know this was said, I'm reiterating it for emphasis). If you would consider a circuit breaker, you should consider a regular time delay fuse, especially since you already have the fuse block.

By the way, the NEC does not tell you to protect the heater at 125% current, it tells you to size the conductors for 125%, and the MAXIMUM size of circuit protection device needs to be capable of protecting those conductors. But there is nothing in the NEC preventing you from using a protective device LOWER than the rating of the conductors.


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

I'm still struggling with the concept of "Protecting the heater".
Heaters sometimes fail. When the fuse blows it's too late. The heater has failed. Period. A new fuse will blow to prove that the heater has failed. The fuse protects the wiring and the system, not the heater.
Why does the fuse fail? If the fuse fails and the heater is good, check the integrity of the fuse holder and the connections to it. A loose or dirty fuse holder or loose or dirty connections to the fuse holder will cause a fuse to "fuse" prematurely.
If you are paying extra for quick acting fuses it may be a waste of money.
Dual element fuses may also be a waste of money. They are very good for motor loads but the ratio of cold current (inrush?) to hot current is so much less in a heater than in a motor that it may be pointless.
Question: What type of heater failure will be disconnected by a fast acting fuse and then what will you do to repair the "saved" heater before re-fusing and re-energizing?
Chronic fuse blowing:- Loose or dirty fuse holder hardware.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racookpe1978]

Can you sell enough control package "upgrades" to make a sensor package change profitable?

You've got enough d**b customers trying to run their heaters in very cold water and without any water flow at all that the heaters are melting out. Blows out the heater, stops their production, gives you a bad name for reliability, etc.

So an upgrade control package would change your current quick-acting fuse (which does nothing to protect the heater) to the CB recommended above, provide a flow sensor for the process fluid (maybe a level detector in the tank, a flow detector on the supply pipe or sensor to read the current in the supply pump, or a valve position sensor on the supply fluid cutout valve, something like that - or a combination of safety trips like those) and a "hot protection" trip that trips the CB when the tank fluid near the heater gets above 250 F. (That is, above the expected 250 degree fluid temp plus a control setpoint margin of 25 to 50 degrees.)

Those trips should stop the heater from melting out. A CB can be controlled to trip out on different signals before the heater melts. A fuse? No protection, only reaction.

 

[robbm]

jraef,
while the NEC doesn't require the 125%, UL508A 31.6.1 says the load shall have branch protection of at least 125% of the heater load, and not larger than the field wiring ampacity rating. sounds like we are just going to use UL489 breakers from now on.

racookpe1978,
these units are sold as the cheap, non-automated equipment, so that's the reason we don't have any protection on them. on units with PLCs, etc, we have those protective design methods in place.

thanks again for all the comments, much appreciated.