Duct heater smoke

[Drew]

Hi all,

I was called to a site where there had been a fire alarm and evacuation after smoke was smelt.
I found a three phase contactor that fed a duct heater bank fused in . The fan had turned off and this one stage got hot and caused smoke. The safeties had kicked in , but due to the fact that the contactor had fused nothing turned the heater bank off.

We isolated the system and ordered new stats,APS, etc.....
ive checked the heaters to not be down to earth using a mega at double applied voltage and they show to be clear.

The strange thing is I've found the fusses to be blown.
the sequence of events could be:
A) fused contactor
b) fan stop
c) elements get hot
D) fusses blow?

Does a heater draw high amps if it gets hot? The heaters when working draw 24amps with a 35 amp fuse. I'm trying to figure out why the fusses blew if the elements are not down to earth? Or do they go down to earth if they get too hot and when cool they are clear?

[chemi-cool]

electric heaters can draw high amps, depends of the design.
There should be fire thermostat just above the heaters to stop electric supply in case of overheating.
You should check the control circuit.

[Drew]

The safeties had kicked in ( air pressure switch, overheat stat,) ,but because the contractor had fused the elements were direct on line . The fire alarm had dropped out the control circuit, but again , with the contractor fused in nothing would drop it out.
The blown fuses saved the day, but I'm wondering how they blew? , because they aren't down to earth while cool.( x 4) I've ordered more fuses and might see issues when I heat up the elements again?

[cadwaladr]

Sounds like you need to update the safety on that system,smoke detection is a priority to shut off the incoming power to the heater contactor,if it catches fire because of overheating then you could have a disaster.

[Drew]

The smoke detection had tried to drop out the contactor but due to the contractor being fused it wouldn't deenergise. In the power circuit there is just the fuses and contactors in line . The contactor is driven by the stat , overheat stat, air pressure switch , fire detection etc... The only way to de energise the system if the contractor is fused is to pull the fuses or to shut down the mechanical services msb.

[frank]

Use 2 contactors in series to prevent a re-occurance

[Drew]

There has been talk of doubling up.
I'm just wondering why the fuses went in the first place.