Solidly Grounded WYE Source

I have a new machine coming that requires ~3 X 480V/277V 60HZ AC. They have also asked for 3 WIRE + GRD (SOLIDLY GROUNDED WYE SOURCE ONLY). What do they mean by solidly grounded in this case?

kckku said:

I have a new machine coming that requires ~3 X 480V/277V 60HZ AC. They have also asked for 3 WIRE + GRD (SOLIDLY GROUNDED WYE SOURCE ONLY). What do they mean by solidly grounded in this case?

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I looked at the equipment schematics. It doesn't need line to neutral voltage so no neutral wire is required.

What I am not sure is if I need a dedicated transformer for this machine? If I have an existing transformer that already has a neutral wire, is it still useable? Meaning I just don't need to connect the neutral for this machine? I would then just connect the three phases and the ground?

A few things here
277V implies there is a natural, because you can’t have 277V without a natural
Leg to leg voltage would be 480V
I don’t know about up north of the US but here all sources must be grounder, the secondary of a transformer is considered as a source. On a 480 / 277 transformer the 277V is a WYE connection the center of the 3 coils is considered the natural .
The statement “3 WIRE + GRD (SOLIDLY GROUNDED WYE SOURCE ONLY)”
Indicated they require the natural be solidly bonded to the earth ground (as per code)
I think they want to ensure that potential between the 3 phases and phase to ground are the correct voltage and balanced

Originally posted by kckku:

What I am not sure is if I need a dedicated transformer for this machine? If I have an existing transformer that already has a neutral wire, is it still useable? Meaning I just don't need to connect the neutral for this machine? I would then just connect the three phases and the ground?

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Based solely on the specification you do not need to run the neutral. You can run just the three phases and a ground. You just need to make sure that the source is from a transformer with a wye secondary and that the X0 point of that secondary is tied to ground. Quite often (but not always) if a machine supplier needs the neutral they will say the machine needs a 5-wire supply.

There are a couple main drivers of this. Many modern drives are designed for a grounded neutral wye supply. Care must be taken when any other supply type is used because the phase to ground voltage may exceed the voltage rating of some of the components in the drive. The other class of components that fall into the same situation are circuit breakers. Many of them are rated 480/277, meaning they can only be used with grounded neutral wye systems for the same phase-to-ground voltage reason.

Keith

I hate to keep coming back to this but as I said before 277V 3phase requires a natural, there is no exception to this.
It’s clear that what they want. If you have an existing 277V supply the can accommodate the increased load then than yes you don’t need to add an additional transformer. If it were my call I would install what they want and avoid problems.
Without the natural you just have 480 t0 480 3 phase isolation transformer
You talked about a 5 wire supply
Can you provide a link to show one or draw it out for me
Keep in mind that the ground connection is never considered as a service / supply conductor
There should never be any current on the ground connection under normal conditions. The only purpose for the neutral to bonded / grounded it insure that the potential remains constant and within a safe range.
According to UL code The neutral conductor is to bonded to ground only at the source and the ground conductor connected to all devices weather they have a natural conductor or not
Over the years I have worked on many services for 120V single phase – 230V single phase ll the way up through 66KV Dual bank substations none of them have even been 5 wire.
I think what you are calling 5 wire service is a Scott – T connection

Scott T Transformer – Voltage Disturbance (voltage-disturbance.com)

I as used and may be still used primarily on the eastern US to feed plants that were originally wired for 240V 2 Phase, do not confuse that with 120V – 230V Single phase they are not the same.
I see people mix this up all the time and it leads to many problems in the equipment.
Also keep in mind outside the US some places don’t require the bonding or even use a neutral at all.
The fact that the company emphasis this tells me that they have had problems with this before and they are trying to prevent problems

GaryS said:

A few things here
277V implies there is a natural, because you can’t have 277V without a natural
Leg to leg voltage would be 480V
I don’t know about up north of the US but here all sources must be grounder, the secondary of a transformer is considered as a source. On a 480 / 277 transformer the 277V is a WYE connection the center of the 3 coils is considered the natural .
The statement “3 WIRE + GRD (SOLIDLY GROUNDED WYE SOURCE ONLY)”
Indicated they require the natural be solidly bonded to the earth ground (as per code)
I think they want to ensure that potential between the 3 phases and phase to ground are the correct voltage and balanced

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The schematic that the machine builder provided only shows the three phase power being used for 480V to 24 VDC power supplies. The three phases aren't being used elsewhere for line to neutral voltage. It's confusing then that they are saying 277V but the machine actually doesn't need it.

I am taking this then as no need to wire to the neutral at the secondary side of the transformer. The X0 point for the secondary side (WYE) needs to be tied to ground.

I would give them what they want if you don't, and things go wrong or there is a problem
there would be no warranty coverage

As kamenges said above, it's likely that there are components, such as VFD's or Power Supplies that have line filtering to ground, and the components are not meant to float above the nominal 277v. I've seen many failures of these filters on 480v delta systems, whether corner grounded or open. Even with an impedance ground system, the filters are not happy on delta power.

I recently had a customer with an older plant that had NO Grounds run to control cabinets, and had numerous faults on the power supplies and drives after an upgrade. The older stuff didn't mind, but the newer components were much more finicky.

Originally posted by GaryS:

You talked about a 5 wire supply
Can you provide a link to show one or draw it out for me
Keep in mind that the ground connection is never considered as a service / supply conductor

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While it is semantically correct to state the ground isn't part of the service, colloquially a 5-wire supply describes a supply with three 3-phase power conductors, a neutral conductor and a ground conductor. Granted, I certainly haven't met everyone in the world. But I believe GaryS is the first person with knowledge of electrical systems that didn't know what I was referring to when I stated 5-wire supply. In the circles I run in that is a pretty typical way of describing a 3-phase supply with neutral and ground.

Keith