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Old 08-18-2007, 02:33 AM   #1
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Default Grounding and bonding a transformer

I have a question about grounding and bonding;

A client has a 120/208 volt service, and he purchased a 480 volt component saw. These saws have anywhere from four to sixteen motors on board, but generally the four or five saw motors are 5 to 7.5 HP, with one saw rated at 10 HP.

To resolve the problem a standard 3 phase transformer was purchased with a Y secondary, and a Delta primary. This transformer was connected backwards with the 208 supplying input to the Y configuration without a neutral. The 480 was taken off from the Delta configuration to supply the safety switch for the machine.

This was wired by the truss plants maintenance staff, however they contacted me regarding the grounding and bonding.

The advice that I gave was to bond the X0 terminal to the transformer frame, and then to carry it to a ground rod, and also to the machine.

Re-thinking this advice, I feel I was mistaken. As the 208 feed is bonded at the service, the XO should NOT have been bonded to the transformer frame.

However the derived system would require that the transformer frame, and the machine to be connected to a ground rod to provide a path for fault current should a fault present itself, so that the supply fuses would open.

The question is, what is the correct way to bond this machine so that it is safe?

Thanks for any advice. Jim


Last edited by Jim; 08-18-2007 at 02:35 AM.
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Old 08-18-2007, 06:54 AM   #2
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Connect transformer chassis to Grounding electrode, and ground 1 phase of the secondary. (So you have a corner grounded Delta)
You also need an Equipment grounding conductor run with the transformer feeder.

I think you would have been better off with a Delta/Delta. I recall people discussing problems with a Wye primary.

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Old 08-18-2007, 07:01 AM   #3
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Quote:
Originally Posted by Jim View Post

Re-thinking this advice, I feel I was mistaken. As the 208 feed is bonded at the service, the XO should NOT have been bonded to the transformer frame.

However the derived system would require that the transformer frame, and the machine to be connected to a ground rod to provide a path for fault current should a fault present itself, so that the supply fuses would open.

The question is, what is the correct way to bond this machine so that it is safe?
The ground rod has nothing to do with fuse operation.

The transformer should be connected the building grounding electrode. (Building steel ot water). The connection from the equipment grounding conductor to secondary is what will operated the OCPD.
You need an EGC run with the feeder. This will operate the primary OCPD in case of a primary fault. GEC bonded to chassis, and secondary, and secondary side EGC will operate the OCPD in case of a fault on the secondary side.
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Old 08-18-2007, 08:17 AM   #4
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1. Check that the transformer manufacture list there transformer for "REVERSE" operation.
2. IMO and I have strong opinions on this based upon many experiences with this type of installation. GET THE RIGHT TRANSFORMER, 208 delta to 480/277 Wye. Avoid future confusion and issues.
3. DO NOT BOND THE XO, DO NOT.
4. Bond all components of the 480 distribution and ground/bond one leg of the 480 as noted to building steel, cold water, this will facilitate operation of the over current protection device. Without this connection you have an ungrounded system which is OK per the NEC if you install fault monitoring.
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Old 08-18-2007, 01:40 PM   #5
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I stand corrected; don’t know what I was thinking. The idea of grounding X0 on the input should have raised a red flag large enough for a banquet table. The ground rod is unnecessary as well because the grounding conductor is most certainly routed with the 3 phase input conductors. The rod would only serve as a redundant ground.

Grounding one corner of the secondary (derived system) 3 phase 480V delta is referenced in 250-26(4), and I was wondering if there were any further references for that specific application.
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Old 08-18-2007, 08:08 PM   #6
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http://www.kilowattclassroom.com/Arc...WYEPhasors.pdf

http://standards.ieee.org/colorbooks.../Greenbook.pdf

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