Connecting X0 on an isolation transformer to ground.

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pnovak22

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Hello everyone,

I am new to this forum and have a question which probably has been discussed/addressed in a previous forum. If that is the case I apologize for the redundancy. It pertains to grounding terminal X0 on an isolation transformer. The application I am referring to is whereby an isolation transformer secondary (75kva, 460V delta primary/240V wye secondary) is connected to a static SCR elevator drive controlling a DC motor. The drive manufacturer suggests not to connect X0 on the secondary of the isolation transformer to ground. The electrical inspector insists that X0 needs to be connected to ground as per NEC. Looking at the NEC code I see section 250.20(B) alternating current systems of 50 volts to 1000 volts that supply premises wiring and premises wring systems shall be grounded based on the following conditions......then looking at 250.21(A)(2)(3)(a-c) indicates the following ac systems of 50 volts to 1000 volts shall be permitted to be grounded but shall not be required to be grounded and it appears 250.21(A)(2)(3)(a-c) applies to my application. The way I read this is that you can ground the X0 but are not required to. It discusses separately derived systems whereby the NEC mentions two of the most common sources of separately derived systems in premises wiring are transformers and generators. BTW, the code I am referencing is 2008 which is currently adopted where I work. I am sure I am overlooking something. Any insight would be greatly appreciated. Thank you.
 
From your description, it doesn't sound to me as if the sds is used "exclusively for control circuits", but maybe I'm misunderstanding your layout.
 
Assuming this is the power circuit for the DC drive, you must EITHER ground the circuit (with the X0 or corner ground one phase), OR add a ground fault monitoring system to it. Grounding is cheap, GFM is expen$ive.

Despite what they told you in the phone, I would bet that whomever said that was an uninformed phone slave. It’s FAR more likely that they REQUIRE the source to be grounded at the X0 to ensure balanced voltages. Every DC drive I have installed required that the source be grounded. They sell what are called “drive isolation transformers” specifically for this purpose, where the input is 240V delta and the output is 240Y139, specifically BECAUSE the drive needs a grounded source.
 
You must ground the X0 of your 240V Wye secondary as it will result in a line to ground voltage of less than 150V, per 250.20(B) in 2014. You would have the option of leaving it ungrounded only if the resultant L-G voltage was greater than 150V. This code change occurred some 30 years ago but it is often overlooked.
 
Maybe they mean that X0 should not be connected to the drive itself, but they are not necessarily excluding X0 from being bonded to ground.
The fact that the disconnect and isolation transformer have labels saying "provided by others" and "fuses or breaker by others" suggests that the schematic for those items may not be definitive and have all of the details necessary for the installation.
 
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