Double fusing for equipment connected via plug

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Sustr

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Dear all,

I tried to find information that can help me to determine what NEC says about overcurrent protection for equipment connected to wall by plug.

1. Lets say we have table standing machine (used for industry and also laboratory) connected to 1-phase power (phase conductor, grounded neutral conductor and grounding protective conductor) with motor and control circuit inside. Motor is connected to mains by contactor. Machine is connected to wall by plug (Nema 5-15). It is not portable but it can be moved to another place.

Overcurrent protection inside equipment - fuses:
A. One fuse in "hot" line of course according wire ampacity, load, etc..

B. Could there be one separate fuse in L and second in N? Same characteristic, current value, two separate fuse holders, etc. In my opinion NFPA 70 and NFPA 79 says that Grounded conductor (neutral) cannot be disconnected by overcurrent device unless all hot conductors are disconnected simultaneously and this cannot be achieved by two separate fuses? You can say that in case of failure N fuse will blow and hot line will stay intact, therefore it can be dangerous situation mainly for service. But if the equipment is connected to wall by plug how can we be sure that L and N are not swapped before equipment? It is etalon that this cannot happen? Therefore solution with one fuse in hot line could be potential dangerous?

Is there any directive/NEC standard that cover this situation. Fuses in both lines (one phase supply) in equipment connected to wall by plug? Or what NEC say about swapped hot and neutral before equipment and one fuse?

C. To cover all of these issues it seems best solution to use circuit breaker with tight handles for L and N. But I am really interested in fuse solution. :)

2. Let?s say we have table standing machine (used for industry and also laboratory) connected to 2-phase power (phase conductor L1, phase conductor L2 and grounding protective conductor) with motor and control circuit inside. Motor is connected to mains by contactor. Machine is connected to wall by plug (Nema 6-15). It is not portable but it can be moved to another place.

Same question - how can be overcurrent protection realized? Could this be done only by circuit breaker with tight handles? Or could fuses be anyhow used to do overcurrent protection of this equipment?


I know that I asked a lot of question in this post, but could You be so kind to give me some reasonable answer/link/directive numbers/articles that describing how to proceed with this?

Many thanks
 
I don't know if this helps any, but keep in mind the NEC generally only covers wiring up to the outlet that supplies the utilization equipment. As you noted NEC 240.22 prohibits overcurrent protection in the grounded conductor (with exceptions including opening all conductors). The equipment itself is normally wired to standards other than the NEC which may or may not address the issue the same way.

ON your Question 2: 240.21 of NEC requires overcurrent protection in each ungrounded conductor.
 
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Sustr said:
Dear all,

B. But if the equipment is connected to wall by plug how can we be sure that L and N are not swapped before equipment? It is etalon that this cannot happen? Therefore solution with one fuse in hot line could be potential dangerous?
Click to expand...

Why couldn?t your use of a polarized Plug and receptacle keep the fuse on L1 and not the neutral
 
augie47 said:
I don't know if this helps any, but keep in mind the NEC generally only covers wiring up to the outlet that supplies the utilization equipment. As you noted NEC 240.22 prohibits overcurrent protection in the grounded conductor (with exceptions including opening all conductors). The equipment itself is normally wired to standards other than the NEC which may or may not address the issue the same way.

ON your Question 2: 240.21 of NEC requires overcurrent protection in each ungrounded conductor.
Click to expand...


I am litte confused from first part. NFPA 79 (Electrical Standard for Industry Machinery) does not belong under NEC?

240.21 request overcurrent protection, but could thes be done by two separate fuses? Or only by tied circuit breaker?
 
david said:
Why couldn?t your use of a polarized Plug and receptacle keep the fuse on L1 and not the neutral
Click to expand...


I was asking for US - can I be 100% sure that phase and neutral are not swapped in wall plug (house instalation)?
For example here in EU in same cases it is possible that phase and neutral can be swapped in wall. Therefore polarized plug and receptable will not help.
 
Sustr said:
NFPA 79 (Electrical Standard for Industry Machinery) does not belong under NEC?
Click to expand...

NFPA70, the NEC, and NFPA79 are two separate and independent standards.
They both deal with electrical safety, so there are some common requirements, but in general they serve two different purposes.
 
In the US plugs are generally polarized so there is no chance of plugging them in backwards, short of someone doing something they should not.

Normally, one would not open the grounded conductor with an over current device in a 120V 2 wire circuit.

In some respects, the answer to your question is not actually answerable based on the information you have provided.

It is quite possible that NFPA79 does not apply to the product you are asking about. In any case, there is no law in the US that says a machine has to meet this standard.

NFPA79 also says this:

1.1.1 The provisions of this standard shall apply to the
electrical/electronic equipment, apparatus, or systems of industrial
machines operating from a nominal voltage of 600
volts or less, and commencing at the point of connection of
the supply to the electrical equipment of the machine.
...
1.3.2 This standard shall not apply to the following:
(1) Fixed or portable tools judged under the requirements of
a testing laboratory acceptable to the authority having jurisdiction
...
3.3.55 Industrial Machinery (Machine). A power-driven machine
(or a group of machines working together in a coordinated
manner), not portable by hand while working, that is
used to process material by cutting; forming; pressure; electrical,
thermal, or optical techniques; lamination; or a combination
of these processes. Machine can include associated equipment
used to transfer material or tooling, including fixtures,
to assemble/disassemble, to inspect or test, or to package.
[The associated electrical equipment, including the logic controller(
s) and associated software or logic together with the
machine actuators and sensors, are considered as part of the
industrial machine.]
Click to expand...

It may be that the "machine" you refer to is better suited to some other classification than industrial machinery and more appropriately designed to some other standard.
 
What's wrong with using breakers at the panel to protect the branch circuit conductors and fuses at the motor to meet your other requirements. ( inrush current? ) The socket and plug is your equipment disconnect and guarantees all the conductors are de-energized when servicing the equipment.

You need to study Article 430 of NFPA70.
 
1. Inspection of US wiring usually includes either random or every receptacle device testing with a plug in tester that will indicate reversed hot/neutral.
2. Two breakers with tied handles will not necessarily provide common trip. That is a separate feature.
 
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