Circuits subject to overloading in normal use.

[Jiajun]

I am looking at the fuses used in hazardous area. I am not quite sure the meaning of the term "circuits not subject to overloading in normal use" in 501.105(B)(5) and some whitepaper from mfg, like

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/technical-literature/bus-ele-an-3186-class-1-div-2-fuses.pdf

From my understanding, it looks like motor is one of the equipment that subject to overloading in normal. But I still don't know how to find out if an equipment or circuit is "subject to overloading in normal use". Is there any guideline I can use?

Another question is why "circuits NOT subject to overloading in normal use" need to be preceded by a proper switch? And it seems not being required for motors, appliances or lamps?

Thanks! I am still kind of new to the industry and sorry for some misuse terminologies if any.

 

[Russs57]

I may be way wrong on this.....but i'm remembering something.

I was curious why they used a transformer to step voltage down for a panel mounted voltmeter and scaled the dial on the meter to display a voltage 4 times higher than it actually received. I was told it was done to limit fault current potential. I think that is an example of what you are asking about.....as in the meter circuit would not be subject to overloading.

 

[Jraef]

No, motors ARE subject to overloading under normal use. What they are referring to are things like METERS and relays that have virtually no load on them, but you need the fuse to protect the conductors against a short circuit.

 

[rbalex]

I agree with jraef. Basically, loads that can "stall" or "jam'" and cause overloads (as opposed to short circuits or ground-faults) are the concern. In addition, to those jraef listed, heaters and other loads that are primarily resistive are what are under consideration.

An interesting side to consider is normal is undefined with respect to hazardous locations. See Section 500.8(B)(5) with regard to motors.

 

[Jiajun]

Thanks for the replies. I am guessing it also somewhat relates to the in-rush current. Like cold start of heater and lamp, the resistance of heating elements is much lower than when it is hot.

 

[GoldDigger]

Thanks for the replies. I am guessing it also somewhat relates to the in-rush current. Like cold start of heater and lamp, the resistance of heating elements is much lower than when it is hot.

Actually, the temperature coefficient of resistance of Nichrome (TM) wire is very small, unlike the very high coefficient between room temperature and incandescent operating temperature for tungsten. So, other than the small amount of current for a built-in fan if there is one, the surge for a heater with Nichrome elements will be small.
On the other hand, the inrush for a tungsten filament light bulb can be 6 or more times greater than the operating current. And for a large filament PAR type bulb the arc surge when the filament fails and substantially vaporizes can also be several times the operating current. This mechanism is a common cause of dimmer failure when used with that type of bulb.

 

[rbalex]

Section 501.105(B)(5) has no interest in momentary overcurrents since they will not “normally” generate sufficient temperatures to ignite a potential hazardous atmosphere; however, long term overloads could. It is important to recognize this only applies to Division 2 since a “hazardous atmosphere” is also unlikely. In Division 1, a short circuit or ground fault could ignite a hazardous atmosphere no matter how transient.

NOTE: Overcurrent and Overload, Short circuit and Groundfault are all defined terms. See Article 100.y