Motor overload control contact

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hughb3

Member
Location
Ohio
Re: Motor overload control contact

ICS 2-2000 Part 2 section 7.3 refers to figure 2-7-3 and 2-7-4, if you look at these figures they are power from L1 and L2 not from the secondary of a transformer with a grounded neutral.
 
N

nowaa

Member
Re: Motor overload control contact

I would have to consider the conductor from the coil to the overload contacts as an integral part of the motor starter. Leave it connected as the NEMA starter is shipped from the manufacturer. If NEMA, IEC, and NEC typically show the OL contacts on the grounded (common) side of the control transformer I would think they have their reasons.
I have always been taught and seen the OL contacts on the right side of coil in ladder diagrams.
Just a little variable in this thread. :D As for safety of operator and equipment. According to an article in Industrial Maintenance & Plant Operation I have read that many OEM are going to 24V DC for controls. Maintenance can be performed safely on energized circuits without wearing prescribed safety gear. Also the have built-in current limiting circuits in case of a short circuit, just clear the short and you are running.

andy
 
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ronaldrc

Senior Member
Location
Tennessee
Re: Motor overload control contact

Ed, why would the "control transformer" need to be grounded
Click to expand...
I admit I have wired up a lot of 0 and 00 or small magnetic starters without grounding one side of the control transformer.

But a good reason I can think of is to protect the transformer from smoking in case of a short between the line and metal enclosure.

If you ground one side and put a in line fuse in the other side it will protect the transformer in two ways.

If you don't ground the transformer the transformer is only protected when both secondary lines go together.

If grounded it also protects if fused side goes to grounded side which wouldn't happen until both lines went to ground or metal enclosure.

A couple more reasons to ground one side of the control transformer.

1.-Takes shock hazard out of touching the common side conductor when troubleshooting.

2.-Give a common testing point for troubleshooting.

Ronald :p

[ December 19, 2003, 09:16 PM: Message edited by: ronaldrc ]
 
H

hughb3

Member
Location
Ohio
Re: Motor overload control contact

It seem that the placement on of the overload on the grounded common side of a circuit is an acception. This does not indicate in no way that it is the preferred way in fact if the control wire leaves the cabinate it can not be on the grounded or neutral side. According to NFPA 78 8.4 you should never break the grounded common with a switch or contact.

Out of NFPA 79
8.3 Control Circuits. Control circuits shall be permitted to be
grounded or ungrounded. Where grounding is provided, that side of
the circuit common to the coils shall be grounded Ungrounded control circuits shall be provided with an insulation at the control
transformer if alternating current or at the power supply terminal if
direct current.

monitoring device that either indicates a ground (earth) fault or
interrupts the circuit automatically after a ground (earth) fault.
Exception No. 1: Exposed control circuits as permitted by Section 6.3 shall be
grounded.
Exception No. 2: Overload relay contacts shall be permitted to be connected
between the coil and the grounded conductor where the conductors between
such contacts and coils of magnetic devices do not extend beyond the control
enclosure.
Exception No. 3: Class 2 low-voltage circuits in Article 725 of NFPA 70,
National Electrical Code shall not require insulation monitoring.

8.2.4 Exclusion of Switching Devices. The equipment grounding
(protective) circuit shall not contain any switches or overcurrent
protective devices. Links or plugs in the grounding circuit shall be
permitted if properly labeled or interlocked with the control circuits.
 
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