Is a grounding electrode connection required for an isolation transformer?

[SysEngAce]

We have equipment that can be supplied with a range of 3 phase voltages from 380-480 VAC. There is a power distribution cabinet which has a 12 kVA delta to eye isolation transformer with a multi-tap primary where we connect incoming power. The transformer primary does not require the neutral but we have a neutral available. The secondary supplies 3 phase 230 VAC (line to line voltage) and uses the center of the WYE to create a line to neutral voltage.

My question is can I connect the secondary WYE center to the incoming power feed neutral or if it has to be connected to a grounding electrode as a seperately derived system?

Also, if it is ok to connect it to the incoming power neutral does it make the isolation less effective?

 

[petersonra]

We have equipment that can be supplied with a range of 3 phase voltages from 380-480 VAC. There is a power distribution cabinet which has a 12 kVA delta to eye isolation transformer with a multi-tap primary where we connect incoming power. The transformer primary does not require the neutral but we have a neutral available. The secondary supplies 3 phase 230 VAC (line to line voltage) and uses the center of the WYE to create a line to neutral voltage.

My question is can I connect the secondary WYE center to the incoming power feed neutral or if it has to be connected to a grounding electrode as a seperately derived system?

Also, if it is ok to connect it to the incoming power neutral does it make the isolation less effective?

I don't believe this is a case where you are required to ground the system. You would need some kind of ground detector if you did not ground the secondary though.

If you connected the neutral of the secondary to the neutral of the incoming power it would no longer be an SDS and you would be prohibited from landing a GEC there.

ETA: If the reason you added this xfmr is to serve a drive, it is not unusual for the drive to require grounded wye power so it would need to be configured as an SDS with a GEC brought to the neutral.

 

[SysEngAce]

I don't believe this is a case where you are required to ground the system. You would need some kind of ground detector if you did not ground the secondary though.

If you connected the neutral of the secondary to the neutral of the incoming power it would no longer be an SDS and you would be prohibited from landing a GEC there.

ETA: If the reason you added this xfmr is to serve a drive, it is not unusual for the drive to require grounded wye power so it would need to be configured as an SDS with a GEC brought to the neutral.

Thanks for the reply Bob.

We do require the neutral on the secondary and there are a number of items connected to the secondary.

My question is if it is required to connect the isolation transformer wye center point to a GEC and GE for the neutral or if it can be connected to the incoming power neutral?

Also, if it is allowed to connect the neutral of the incoming power then are there any features of having the isolation transformer lost when doing this or does it perform exactly the same in either case? Another words, are some of the features of harmonic transmission blocking or other noise transfer characteristics lost if the incoming neutral is connected?

 

[Smart $]

Thanks for the reply Bob.

We do require the neutral on the secondary and there are a number of items connected to the secondary.

My question is if it is required to connect the isolation transformer wye center point to a GEC and GE for the neutral or if it can be connected to the incoming power neutral?
...

You have to connect a GEC run to nearest: water pipe electrode or structural steel electrode. If neither of those exist, any other electrode identified in 250.52(A) [NOTE: it has to be one that is connected to the primary system.]

That is, assuming you do not connect X0 to the primary neutral. While Code does not explicitly say you cannot, there is Code that when correlated says you cannot do that... but it is far from explicit, having to integrate the rationale behind several requirements... just can't recall which ones they are off hand. :happysad:

 

[ActionDave]

Thanks for the reply Bob.

We do require the neutral on the secondary and there are a number of items connected to the secondary.

My question is if it is required to connect the isolation transformer wye center point to a GEC and GE for the neutral

250.20 and 250.21 is your code reference.

or if it can be connected to the incoming power neutral?

From what I understand you don't want to do this, it causes problems with heating in the transformer. I have no direct experience with this because I have never done nor seen an install where the primary neutral was connected to the secondary.

Also, if it is allowed to connect the neutral of the incoming power then are there any features of having the isolation transformer lost when doing this or does it perform exactly the same in either case? Another words, are some of the features of harmonic transmission blocking or other noise transfer characteristics lost if the incoming neutral is connected?

If you splice together wiring from your primary to your secondary you loose all hope of isolation.

 

[steve66]

Thanks for the reply Bob.

We do require the neutral on the secondary and there are a number of items connected to the secondary.

My question is if it is required to connect the isolation transformer wye center point to a GEC and GE for the neutral or if it can be connected to the incoming power neutral?

Also, if it is allowed to connect the neutral of the incoming power then are there any features of having the isolation transformer lost when doing this or does it perform exactly the same in either case? Another words, are some of the features of harmonic transmission blocking or other noise transfer characteristics lost if the incoming neutral is connected?

Yes, I think you would loose some of the benefits of an isolation transformer by connecting the center of the wye back to the input neutral. The input side wouldn't normally even have a neutral since its a delta input.

I think the normal route would be to treat it as a SDS, and ground the center of the transformer secondary.

I would not connect the output back to the input without a very good reason. That could also cause problems with any in-balances in the source or load.

 

[junkhound]

does it make the isolation less effective?

- yep, can render the isolation useless.

You DO NOT want to connect the neutrals if you want full isolation xfmr capability.

In 1974, we retrofitted the power system and installed 45 kVA delta-wye transformers in the missile silos at Grand Forks.
The neutrals were SPECIFICALLY isolated, both for high frequency and harmonic reasons, ONLY the 3 phase lines went to the silo. Needed to survive hypothetical USSR first strike.

Analysis showed that if the neutral were connected, the missile electronics would be fried by NWE (talking hundreds of kA induced on external power lines and sub Hz to many MHz frequency content). The secondary neutral was connected to the 80 foot deep steel silo liner.

All moot now, all Grand Forks silos were imploded as part of Start Treaty; however, still cannot share any additional details for another 35 years or so.

 

[SysEngAce]

Thanks all!

I'm still interested in if anyone knows the specifics in the code which would indicate this as a violation?

I had failed to find anything indicating the incoming neutral couldn't be connected. I wasn't clear if connecting it would deem it not a separately derived system and thus not being subject to the SDS requirements or if an isolation transformer is always considered an SDS.

The application is going from a fixed site for medical imaging equipment to a mobile trailer. When the trailer is at a site such as a hospital the power is changed from generator power to being plugged into the hospital. Having to deal with the GEC and GE poses additional complications and is why I'm investigating this alternative. It does make sense that connecting a wire from the power source to the secondary would likely reduce/eliminate the benefits of the isolation transformer but once again I don't have any experience doing so and wanted to learn if this type of thing is done.

One further note: The reason we have the incoming neutral pulled into the equipment isn't for the delta-wye isolation transformer (obviously, it isn't used for it) but because the neutral is used for other parts of the system.

 

[ActionDave]

I'd bond the wye secondary to the frame of the trailer and call it good.

 

[dionysius]

We have equipment that can be supplied with a range of 3 phase voltages from 380-480 VAC. There is a power distribution cabinet which has a 12 kVA delta to eye isolation transformer with a multi-tap primary where we connect incoming power. The transformer primary does not require the neutral but we have a neutral available. The secondary supplies 3 phase 230 VAC (line to line voltage) and uses the center of the WYE to create a line to neutral voltage.

My question is can I connect the secondary WYE center to the incoming power feed neutral or if it has to be connected to a grounding electrode as a seperately derived system?

Also, if it is ok to connect it to the incoming power neutral does it make the isolation less effective?

The Delta primary side has no neutral.
The secondary Wye neutral is bonded to the same ground as the primary supply power using a GEC.
The primary and secondary neutrals are never connected together to preserve isolation.

 

[iwire]

The Delta primary side has no neutral.
The secondary Wye neutral is bonded to the same ground as the primary supply power using a GEC.
The primary and secondary neutrals are never connected together to preserve isolation.

Well not directly connected but they are connected through the GES. There is no true isolation of a neutral.

 

[dionysius]

Well not directly connected but they are connected through the GES. There is no true isolation of a neutral.

That is correct. Sometimes we and the code gurus are silently trying to elude ourselves into thinking what we want to believe. If you carefully examine most of the statements even here on this thread you will find some seductive thinking. Mother Nature however is fair......she seeks out the lowest resistance path regardless of our interventions.

 

[iwire]

Mother Nature however is fair......she seeks out the lowest resistance path regardless of our interventions.

I would say that is more selective thinking.

She seeks out, and uses any and all paths.

 

[dkidd]

trying to elude ourselves

I've never been successful at eluding myself. No matter where I go, I'm right there.