NFPA 79

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smallz

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We have a machine that we installed a non fused disconnect below the disconnect we have a terminal strip to feed several breakers that feed equipment on the machine. Do I need over current protection between the disconnect and the breakers.


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What's feeding the disconnect switch? What size wires run out to the downstream circuit breakers? What's the distance between the disconnect and those breakers? Why did you put NFPA 79 in the title? Is it an industrial machine?
 
100 amp breaker feeding disconnect. 3 awg from disconnect and distribution block. 3 feet of wire. It is in a cabinet for a industrial machine.


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smallz said:
We have a machine that we installed a non fused disconnect below the disconnect we have a terminal strip to feed several breakers that feed equipment on the machine. Do I need over current protection between the disconnect and the breakers.


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What is the available fault current? If the available fault current exceeds 5 (or maybe it is 10) kA you will probably need appropriate fuses to protect the switch and terminal blocks.
 
smallz said:
It is supplied by a 100 amp breaker 240 volt 1ph


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probably is not listed by UL to protect both the terminal blocks and the switch, so you would have to hope the available fault current is below the SCCR of those devices.
 
smallz said:
They are rated for 175 amps


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sccr = short circuit current rating.

everything in a power circuit is required to be able to accept the maximum available short circuit current.

Short-Circuit Current Rating. The prospective symmetrical
fault current at a nominal voltage to which an apparatus
or system is able to be connected without sustaining damage
exceeding defined acceptance criteria.
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110.10 Circuit Impedance, Short-Circuit Current Ratings,
and Other Characteristics. The overcurrent protective
devices, the total impedance, the equipment shortcircuit
current ratings, and other characteristics of the
circuit to be protected shall be selected and coordinated to
permit the circuit protective devices used to clear a fault to
do so without extensive damage to the electrical equipment
of the circuit. This fault shall be assumed to be either between
two or more of the circuit conductors or between any
circuit conductor and the equipment grounding conductor(
s) permitted in 250.118. Listed equipment applied in
accordance with their listing shall be considered to meet the
requirements of this section.
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409.22 Short-Circuit Current Rating. An industrial control
panel shall not be installed where the available fault
current exceeds its short-circuit current rating as marked in
accordance with 409.110(4).
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670.5 Short-Circuit Current Rating. Industrial machinery
shall not be installed where the available fault current exceeds
its short-circuit current rating as marked in accordance with
670.3(A)(4).
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Most switches, terminal blocks, and power blocks are rated for no more than 10 kA of short circuit current. Many are listed for higher values if protected by specified fuses.

It is generally an OSHA requirement that every electrical panel be labeled with the maximum short circuit current that might be seen there. Whatever devices you feed there have to have a SCCR rating higher than the available short circuit current.
 
He's right, you need to start there and work backward. Your machine is now SUPPOSED to show an SCCR on it's nameplate. If none is shown, the "courtesy" rating of an untested system is 5kA. If you can prove there is 5kA or less Available Fault Current at the terminals of the disconnect switch, you are good to go. If not, you have a bigger problem.

It's unfortunately all too common for machine builders to totally ignore this issue, because the onus falls on the INSTALLER. So rather than go to the expense of designing their control panel in a way that nets them a higher SCCR, they ignore it, or actually just put the courtesy rating on their nameplate and ship it, which means they are just kicking the can down the road to you. You might be able to deal with it by using current limiting fuses, i.e. use a fused disconnect instead of a non-fused (even though you also have a breaker), but that often has to be a study performed by an Engineer in order for an AHJ to accept it.
 
Jraef said:
It's unfortunately all too common for machine builders to totally ignore this issue, because the onus falls on the INSTALLER. So rather than go to the expense of designing their control panel in a way that nets them a higher SCCR, they ignore it, or actually just put the courtesy rating on their nameplate and ship it, which means they are just kicking the can down the road to you. You might be able to deal with it by using current limiting fuses, i.e. use a fused disconnect instead of a non-fused (even though you also have a breaker), but that often has to be a study performed by an Engineer in order for an AHJ to accept it.
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We don't ignore it. We ask them and they virtually always tell us they don't know. We end up telling them anything above 5 kA is a little more and they say just do what is cheapest.

It is like when I use the type E and F motor starters. I tell them they can only be used on 480 wye systems. Most of our customers do not seem to know what the power system is in their plant and it confuses them. it is why I have sort of gone away from these devices unless I can convince myself the power really is 480 wye.
 
smallz said:
Looking for some training on NFPA 79. Any suggestions?


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Check with local wholesalers around you who may sponsor a seminar. I've got an 8 hour ceu class on nfpa 79 next month that is put on by van meter. We do a couple every year
 
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