Purge Type X - Cool Down Time For Braking Resistor

cowski

Member
Location
New York
Occupation
Engineer
I'm planning to place an Allen Bradley VFD and braking resistor in a Type X purged enclosure. The area is Class I Div 1, Class 2 Div 1.

The braking resistor never seems to get hot during normal operation, but I know it has a safety temperature switch that opens at 227 degrees C. I am concerned that the braking resistor could heat up during an anomaly (say the machine is overloaded) and pose a threat if the panel is opened.

Do I need to place a "Warning: High Temperature Internal Parts" label on this box and use exception 4.6.1 (1)? How would I determine the amount of time needed for the braking resistor to cool down? Is the fact that Allen Bradley built a temperature switch into the brake proof enough that it may overheat during "normal operation?"
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
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Professional Electrical Engineer
I'm assuming you are citing NFPA 496 for the Exception.

I would suggest reviewing several NEC Sections: 500.8(A)(3), 500.8(B)(5) and 500.8(C)(4). Section 500.8(A)(3) would indicate seeking the manufacturer's determination of cooldown. Section 500.8(C)(4) might surprise you. The Class I "T-Code" is fairly low. Class II could be a problem.
 

cowski

Member
Location
New York
Occupation
Engineer
Thank you Bob. You're correct that I was referencing NFPA 496.

It looks like the 227°C trip point would be a Class I temperature code of T2C. I will check with Allen Bradley as you recommend.

The Class II temperature code mandates operation with the maximum amount of dust blanketed on the outside of my purged enclosure, right? (per NEC 500.8(C)(4)) So it would reduce the cooling rate of components inside.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I have seen a few cases where they have put a sign on the enclosure that says something like "do not open unless area is known to be nonhazardous". As part of the hot work permit they are required to have some kind of monitoring to prove a hazardous condition does not exist prior to opening the door.

AB put the switch there to protect the resistor from overheating due to people who do not properly size them.

You might be able to have a window that would allow an IR T/C to 'see" the resistor to determine its temperature prior to opening.

Or bolt a T/C to the resistor so the temperature can be measured.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
I'm planning to place an Allen Bradley VFD and braking resistor in a Type X purged enclosure. The area is Class I Div 1, Class 2 Div 1.

The braking resistor never seems to get hot during normal operation, but I know it has a safety temperature switch that opens at 227 degrees C. I am concerned that the braking resistor could heat up during an anomaly (say the machine is overloaded) and pose a threat if the panel is opened.

Do I need to place a "Warning: High Temperature Internal Parts" label on this box and use exception 4.6.1 (1)? How would I determine the amount of time needed for the braking resistor to cool down? Is the fact that Allen Bradley built a temperature switch into the brake proof enough that it may overheat during "normal operation?"
You did not mention size and the duty cycle of the motor-- or should I say-- does it require a lot of stop and go operation?

Or-- whether it is intermittent that requires the motor to operate on "hauling" brake mood. . . meaning applying a certain drag on the motor to get the precise speed.
In some application the motor is allowed to coast to stop.

Any of these conditions has influence on how your brake is applied. If the brake resistor came with the VFD, I would trust Allen Bradley had it properly sized.
Most brake engagement lasts between .5 to 5.0 sec for medium sized VFD. For near-instantaneous stop, it is common to come to stop at the same duration it takes to come up to full speed.
This short stopping (instantaneous) manner doesn't take into account the .5 to 50 seconds allowance the electronic brake engagement duration. It is generally better to hold the brake longer because of the kinetic energy of the driven load.

My comment above is based on your criteria using a brake resistor . . .no dynamic braking involved. The dearth of information you provided is what my response were based on.
No speculation, no hypotheticals and no wild guesses.
So, if it doesn't meet your expectation I'd be glad to accept the shortcoming.

As for placing the resistor inside Type X enclosure-- I would consult the VFD mfg for suitability of its operation in the absence of airflow.
Worst case scenario, the heat generated by a 20 HP motor brake resistor is equivalent to around 4 kilowatts.

Most I had installed were mounted outside the cabinet with fully ventilated cage-like enclosure.
When you say "purged" does it mean you pull the heat out and replace it with makeup [fresh] air ?

How this would play out in the area of hazardous location-- being exposed or intrusion of outside air, I wonder.

When I think of Type X enclosure, it means it is isolated.

Having said that , perhaps you might want to consider mechanical friction brake in a TEFC housing motor or regenerative braking which is accomplished by the DC bus of the VFD itself and all inside the cabinet.

Though it may be possible to have properly sized heat sink and supplementary drain resistor.

As for warning system to monitor heat build-up. . .it seems like a measure to avert something that should not be given a chance to happen in a well designed system.
Failure is not option IMO.

Whether that is feasible, in your situation, only you can determine.

All the best.
 
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