yendor213
New User
- Location
- West Virginia
- Occupation
- Electrician
Hey folks,
We have a job request to run a circuit and hook up a 3ph 480v primary - 3ph 1000v secondary, ungrounded, 300kva xfmer for short duration test equipment on motors for dust extraction equipment. Large hp like 125-150hp for tunneling projects etc.
There is a guy in town who builds transformers and rewinds motors who is designing and building this. No engineer, no blueprints. We are doing a design build for the circuitry.
My question has most to do with grounding/bonding of this ungrounded (no neutral system).
Here's the overall layout to set up the question:
There is an existing CT service on this building. The plan is to come into that existing service and use insulated lugs to run another circuit to a new NEMA 3R 400 amp fused disconnect with 3-500mcm copper. Hots only. From the 400 amp to the primary 480v side of the transformer.
Through the outdoor, drytype transformer it steps up to a 1000v circuit and we are running up an exterior wall into the building and down to the fused disconnect on the equipment using xhhw-2 copper wire. (#1, for 125amp sizing per equipment manufacturer) They will have 100 amp fuses. Approx 140'
So the guy that is building/designing the transformer met with us last week and told us that this will be an "ungrounded system" or no neutral bond. There will be a neutral provided within the transformer "just in case the customer needs it with a replacement piece of equipment", but not bonded to an earth ground. He said he will leave it "floating".
He advised us just to come off of the two existing ground rods for the existing system, run into 400amp disconnect, into transformer, through to final equipment. This earth ground is only to bond the cabinets, no connection/bond to the neutral within the transformer. The equipment coming does not need a neutral.
My understanding of an ungrounded system is that it is used so that if there is a phase to ground fault it wont kill the circuit so that critical/important equipment won't shut off right away like in a foundry or assembly line. This phase to ground fault would become like a "corner grounded system then"?
If this condition goes unnoticed and another fault occurs with another phase to ground, it would actually act or become a phase to phase fault and cause catastrophic failure? i.e. arc flash, blast?
So I guess the question is, should I use the existing ground rods like the man building the equipment advised? I'm concerned that the 'ungrounded' system may be able to "see" the bond to neutral in the existing system and therefore it is already at a 'corner grounded system' condition meaning if it faulted from any phase to equipment ground it would trip the breaker/blow the fuse on the 1000v side? Or am I overthinking it. In a way my earth ground is mated with existing earth ground which is bonded to existing neutral.
I kinda think I should run this as an independent ground rod system away from the other set of rods?
Additionally, should I be concerned with the floating neutral in the 1000v secondary side?
Thank you all for any insight into this.
We have a job request to run a circuit and hook up a 3ph 480v primary - 3ph 1000v secondary, ungrounded, 300kva xfmer for short duration test equipment on motors for dust extraction equipment. Large hp like 125-150hp for tunneling projects etc.
There is a guy in town who builds transformers and rewinds motors who is designing and building this. No engineer, no blueprints. We are doing a design build for the circuitry.
My question has most to do with grounding/bonding of this ungrounded (no neutral system).
Here's the overall layout to set up the question:
There is an existing CT service on this building. The plan is to come into that existing service and use insulated lugs to run another circuit to a new NEMA 3R 400 amp fused disconnect with 3-500mcm copper. Hots only. From the 400 amp to the primary 480v side of the transformer.
Through the outdoor, drytype transformer it steps up to a 1000v circuit and we are running up an exterior wall into the building and down to the fused disconnect on the equipment using xhhw-2 copper wire. (#1, for 125amp sizing per equipment manufacturer) They will have 100 amp fuses. Approx 140'
So the guy that is building/designing the transformer met with us last week and told us that this will be an "ungrounded system" or no neutral bond. There will be a neutral provided within the transformer "just in case the customer needs it with a replacement piece of equipment", but not bonded to an earth ground. He said he will leave it "floating".
He advised us just to come off of the two existing ground rods for the existing system, run into 400amp disconnect, into transformer, through to final equipment. This earth ground is only to bond the cabinets, no connection/bond to the neutral within the transformer. The equipment coming does not need a neutral.
My understanding of an ungrounded system is that it is used so that if there is a phase to ground fault it wont kill the circuit so that critical/important equipment won't shut off right away like in a foundry or assembly line. This phase to ground fault would become like a "corner grounded system then"?
If this condition goes unnoticed and another fault occurs with another phase to ground, it would actually act or become a phase to phase fault and cause catastrophic failure? i.e. arc flash, blast?
So I guess the question is, should I use the existing ground rods like the man building the equipment advised? I'm concerned that the 'ungrounded' system may be able to "see" the bond to neutral in the existing system and therefore it is already at a 'corner grounded system' condition meaning if it faulted from any phase to equipment ground it would trip the breaker/blow the fuse on the 1000v side? Or am I overthinking it. In a way my earth ground is mated with existing earth ground which is bonded to existing neutral.
I kinda think I should run this as an independent ground rod system away from the other set of rods?
Additionally, should I be concerned with the floating neutral in the 1000v secondary side?
Thank you all for any insight into this.
