VSD Installations

[ccov]

Industrial location. 3 phase 480V delta source - 25A Fused Disc (in a remote location, capable of being locked out) feeds a 15KVA delta-whye drive isolation xfmr (18FLA) (#10 wire). The secondary of this xfmr immediately feeds a 10hp VSD (within 10 feet), which in turn supplies the 10hp motor (#10 wire) (in another remote location).

The questions I have are this:
1. Does a disconnect need to be within sight of the VSD?
2. Because the xfmr is fused on the primary @ 125%, it does not require secondary protection. Does the VSD, which provides electronic OL protection, meet the requirements for transformer secondary CONDUCTOR protection requirements?

Thanks

 

[dpc]

1. Yes, unless the VSD has its own disconnecting means. This is required by NEC Article 430.

2. I would say no. There is no way to ensure that the VSD will be programmmed in such a manner that the conductor will be protected within requirements of NEC and no way to visually tell if it remains programmed that way in the future.

 

[LionelHutz]

2. I think code requires secondary protection unless the transformer is single phase without a multi-tapped/multi-winding secondary. I know CEC does but don't know if NEC does. At any rate, most, if not all, small VFD's only measure the output current so they can't provide any protection for incoming cables.

 

[mil3]

ccov,
Yes, code calls for visual disconnect.
and Yes again to your second question. The secondary should be fused along with the primary. Code calls for safety and when in doubt....install it. It is for your protection and safety of the plant and people.
Besides...the cost is marginal for the sizing you are talking about.

 

[jraef]

Even if you had a standard starter, code requires a SCPD on the motor circuit sized to protect the conductors. Even though it appears permissible to allow transformer secondaries to be protected by primary fuses under 240.21(C) via one or more of the exceptions, that really only relates to it as a transformer secondary circuit. For motor circuits, you need to read section 430, specifically 430.102. VFDs have added requirements as well. Read 430.2.

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[ccov]

Thanks all for the replies.
On a side note, can anyone give me a true reason a stand alone VSD needs a disconnect within site? I am talking small (480 - 230VAC < 100hp) VSDs.
In an industrial facility, drives may be located outside the electrical room (to minimize the drive to motor feeder length or simply space constraints).
I see no benefit to this, other than the Electrician or Engineer not having to walk back to the MCC to lock it out!
Having a disconnect within site of the motor does make sense to me, however this requirement can be "excepted" by having a lockable disconnecting means in a remote location. So why not the same exception for the controller (aka VSD)??
Thanks.

 

[dpc]

A true reason? NEC 430.102. The intent is that someone working on the motor controller (VSD) be able to see the disconnecting means that is keeping him/her from being electrocuted. It's a worker-safety issue. If you were able to keep applying that exception, pretty soon no disconnect except a main service disconnect would be required. A local disconnect greatly increases the odds that it will actually be used.

At the end of the day, the logic or lack of logic doesn't matter at all - it is in the NEC so it just is.

 

[jraef]

There is an exception at the end of 430.102(B), wherein the circuit disconnect does not need to be within sight IF it is a controlled industrial facility with established safety procedures, and the disconnect in another room is lockable. They specifically address this as being related to motors 100HP and over where it is not practical to have a local disconnect if one is already present somewhere else. This was new or changed in the 2002 NEC. Keep in mind though that the judgement of the AHJ is final when it comes to the controlled industrial facility part of that. Better to ask first than get nailed afterwards and have to start breaking up concrete.

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[ccov]

In my opinion, 430.102(A) Exception No. 1 should allow all voltage class motor circuits, not just above 600V.

 

[dpc]

You always have the option of asking the local inspector for a variance from NEC requirements in this particular instance.