Delta - Delta Transformer: Grounding

[W@ttson]

Hello all,

I just realized that a Contractor installed a step up transformer 208V Delta to 480V delta. He brings grounds in from the primary feeder and sends ground out with the secondary feeder to an MCC (feeds several motor loads, plus some drives, plus some heating loads).

However, the transformer is not corner grounded, nor is it center tapped. This leaves it being ungrounded. Is it required to be grounded? In that case I assume making it a corner ground system would be easiest?

If it can be ungrounded, I assume a ground fault detector would need to be installed, plus some labeling be installed on the MCC, and then make sure no drives require the MOVs to be disconnected as a result of being connected to an ungrounded system.

Am I missing anything?

 

[electrofelon]

Sounds like you got it all. Not required to be grounded, but a delta -wye probably would have been a better choice.

 

[W@ttson]

Sounds like you got it all. Not required to be grounded, but a delta -wye probably would have been a better choice.

Thank you for the reply.

How would the GF detector work. Typically those devices try to sense the current different between phase conductors, neutral conductors, and grounds (i.e. if current is leaking to ground it will be at an imbalance and trip). If the system is ungrounded and there is a single line to ground fault, essentially the system would become a corner grounded system, no fault current would leak. How would the GFP sense the fault? Will it be reading the voltages too? So that if it senses that one of the phases instead of 480V to gnd it became 0V to ground?

 

[ptonsparky]

Thank you for the reply.

How would the GF detector work. Typically those devices try to sense the current different between phase conductors, neutral conductors, and grounds (i.e. if current is leaking to ground it will be at an imbalance and trip). If the system is ungrounded and there is a single line to ground fault, essentially the system would become a corner grounded system, no fault current would leak. How would the GFP sense the fault? Will it be reading the voltages too? So that if it senses that one of the phases instead of 480V to gnd it became 0V to ground?

They used to use a series of lamps to indicate the grounded conductor.

 

[jim dungar]

Thank you for the reply.

How would the GF detector work. Typically those devices try to sense the current different between phase conductors, neutral conductors, and grounds (i.e. if current is leaking to ground it will be at an imbalance and trip). If the system is ungrounded and there is a single line to ground fault, essentially the system would become a corner grounded system, no fault current would leak. How would the GFP sense the fault? Will it be reading the voltages too? So that if it senses that one of the phases instead of 480V to gnd it became 0V to ground?

On a delta system these are called Ground detectors not 'ground fault'. They are typically voltage sensing devices.

In the simplest system a ground can be detected using lamps. If you connect (3) 480V lights into a wye configuration, with one side per phase and the wye point to ground. Each lamp would glow dimly as it only sees 277V. When a phase goes to ground the lamp connected to it would go out as it's voltage goes to zero, while the other two lamps go to full brightness as the voltage across them rises to 480V.

 

[Birken Vogt]

I've seen a couple of these where the tenant required 480 volt power and the building supplied 208. In both cases they just wired the 208 to the X side and the 480 from the H side of a standard transformer. They may have been ungrounded, I was there looking after the tenants had left. I doubt they were properly engineered or inspected or GFD installed anywhere.

 

[W@ttson]

On a delta system these are called Ground detectors not 'ground fault'. They are typically voltage sensing devices.

In the simplest system a ground can be detected using lamps. If you connect (3) 480V lights into a wye configuration, with one side per phase and the wye point to ground. Each lamp would glow dimly as it only sees 277V. When a phase goes to ground the lamp connected to it would go out as it's voltage goes to zero, while the other two lamps go to full brightness as the voltage across them rises to 480V.

Has anyone seen a commercial product that I can call out say “Eaton model number xyz”?

I have seen those light glowing ones by a company named “Erikson”. I emailed them and the owner actually responded but they use a distributor scheme such that he could not give me any pricing and pawned me off to graybar which I could not get a hold off to get me a quote.

 

[jim dungar]

Google 'ground detectors'. Look for Biddel and Littelfuse.
I sold the Erickson product, decades ago.