Wye Wye Transformer Bonding

[Levin94]

Two things I've always held to:

1) Neutral and Equipment Grounding conductor are bonded at Service panel (point of origination) and NEVER to meet again.

2) With transformers creating a Newly Derived system, Neutral and Equipment Grounding conductor are Bonded at Secondary ONLY.

Have run into a situation with the manufactures diagram/instructions on a wye wye transformer. It is calling for the Primary & Secondary Neutral to be"Tied" together and both Primary & Secondary are both bonded to chassis ground. (equipment ground)
My Boss called the Manufacture and they insist this is correct. To me this seems to go against the concept of an isolation transformer. Is a Wye Wye not an "Isolation" transformer?

 

[jim dungar]

Utility owned transformers are commonly installed as Grounded Wye - Grounded Wye. in fact many padmount transformers have the neutral points bonded together in the tank, with only the X0 bushing being brought out. However, customer owned transformers, which must be installed per the NEC, are rarely installed in this manner. Most customer installations would leave the primary side neutral point (e.g. H0) floating, while the secondary (e.g. X0) is grounded.

Liquid filled transformers with wye-wye transformers can certainly be ordered with no internal bonding between H0 and X0.

 

[GoldDigger]

Utility owned transformers are commonly installed as Grounded Wye - Grounded Wye. in fact many padmount transformers have the neutral points bonded together in the tank, with only the X0 bushing being brought out. However, customer owned transformers, which must be installed per the NEC, are rarely installed in this manner. Most customer installations would leave the primary side neutral point (e.g. H0) floating, while the secondary (e.g. X0) is grounded.

Liquid filled transformers with wye-wye transformers can certainly be ordered with no internal bonding between H0 and X0.

If the secondary is a delta, the primary wye neutral point generally must NOT be connected to neutral conductor in a customer environment, let alone bonded. A POCO can choose (and usually does) to connect the wye point to neutral because they have full control of the balance of the line to neutral voltages.
In a wye-wye, on the other hand, I see no need to float the primary wye point, but if it is connected it must not be bonded to the EGC or case, nor to the secondary wye point if the goal is a separately derived system. Even for an SDS, however, the X0 should be bonded to the GES.

 

[electrofelon]

What are the voltages? I dont see how this would be compliant (as the OP described was being suggested) per 250.24(A)(5) unless you are using the provisions of 250.184(C).

 

[Levin94]

Thanks for all your input here. The voltage is 120/208 To 277/480. I do not think this can be considered a true "Isolation" Transformer, if the neutrals are connected.

 

[electrofelon]

Thanks for all your input here. The voltage is 120/208 To 277/480. I do not think this can be considered a true "Isolation" Transformer, if the neutrals are connected.

I dont think the neutrals being connected is an issue. Any two systems that are both grounded with a transformer between will have the neutral/grounded conductor connected together. To have true isolation, one of the systems will need to be ungrounded. The issue I see is connecting the primary neutral to ground violating 250.24(A)(5).