fusing in C1D2 environments

[jsummers]

I read in NEC2008 section 501.105(B)(5) the following:
"... fuses for overcurrent protection of instrument circuits not subject to overloading in normal use shall be permitted to be mounted in general purpose enclosures if each such fuse is preceded by a switch complying with 501.105(B)(1)."

I don't understand the thought process here and would appreciate some enlightenment. According to the first part of the clause quoted above the fuse is for overcurrent protection for faults that are not part of "normal use". If a fuse blows due to a faulty circuit condition, how does having a switch in front of the fuse make it any safer (from a standpoint of creating or preventing an arc that could cause an explosion)? Unless someone is there to operate the switch how does it help at all?

I would frequently like to fuse my I/O or power circuits in enclosures with low cost, standard fuses. All of the fuse manufacturers say that such fuses don't exist (the operating element either has to be oil immersed or in a hermetically sealed chamber). Does anyone know of a low cost way of providing fusing that still meets the code? (BTW, this question is assuming that all other components in the enclosure are C1D2 rated and I am using a General Purpose enclosure).

Thanks,

JS

 

[rbalex]

You should probably cite the entire sentence; it qualifies the application, "Where general-purpose enclosures are permitted in 501.105(B)(1) through (B)(4)..." In other words, a general purpose enclosure must already be permitted within the scopes of 501.105(B)(1) through (B)(4).

The manufacturers are in error, there is no requirement in Section 501.105(B)(5)for the fuses to be oil immersed or in a hermetically sealed chamber. They are thinking about 501.115(3). Actually, most glass fuses are hermetically sealed, but the manufacturers won't guarantee it and it is irrelevant in this case anyway.

 

[jsummers]

Your are correct sir

Your are correct sir

Your are correct in that I should have quoted the entire clause. It does add context.

However, my question still stands, even in light of the entire context. 501.105(B) specifically refers to C1D2 locations. So, assuming that the requirements of (1) - (4) are met and that the fuses are mounted in a general purpose enclosure, how does having a switch that proceeds the fuse in the circuit make it safer during operation? I understand that the switch can be opened if the fuse is being replaced, thereby de-energizing the circuit, but is that the only concern?

I was looking at it more from the perspective of an abnormal overload event occurring during operation which would cause the fuse to blow as being a potential problem in a hazardous area. Am I making an improper assumption that this is what the upstream switch is supposed to be protecting? Or even an improper assumption that a non C1D2 rated fuse blowing is a (potential) danger in a C1D2 environment.

Thanks for your time in considering this.

 

[rbalex]

... I understand that the switch can be opened if the fuse is being replaced, thereby de-energizing the circuit, but is that the only concern?

Yes

I was looking at it more from the perspective of an abnormal overload event occurring during operation which would cause the fuse to blow as being a potential problem in a hazardous area. Am I making an improper assumption that this is what the upstream switch is supposed to be protecting? Or even an improper assumption that a non C1D2 rated fuse blowing is a (potential) danger in a C1D2 environment.

"Abnormal" conditions generally don't count in Division 2 analysis.