I was asked today if an isolation transformer (30kva, 480 to 480, 3ph, no neutral) needed to have a grounding electrode (building steel in this case)? Or, can the equipment ground be carried through the transformer, primary to secondary, connected to cabinet and XO on the way by? My initial response was no grounding electrode is necessary because there is no neutral derrived. Possibly because I've seen it done like that so many times. However, I didn't like the fact that I couldn't back it up with an NEC reference on the spot. I'm leaning towards 250.30.a.3 exception3. So, now I am doubting my first "knee jerk" answer. I know you guys can help me with this. Thanks.
Isolation transformer grounding question
- Thread starter btharmy
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DARUSA
Senior Member
- Location
- New York City
Ungrounded system ? Is there a GF protection on the secundary side ?
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
Does your system meet the requirements of 250.21 ?
Can you give us a little more detail, because you say there is no derived neutral, but there is an X0.
Is the transformer wye-delta ?
Can you give us a little more detail, because you say there is no derived neutral, but there is an X0.
Is the transformer wye-delta ?
As far as i know, it is delta wye, in a industrial facility w/ "qualified" personnel. It is feeding only a control cabinet for a machine. The cabinet has a fused desconnect and contains dc drives and some control circuits for the machine it serves. That's about all the info I have available to me.
I think it meets the requirements or 250.21 which isn't required to be grounded. It also doesn't seem to fall under 250.20.b.1-3. But, xo should still be bonded which would be sized according to the gec not equipment ground anyway. If there is building steel close, I say gec to it.
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
If it has a WYE secondary I agree with bonding the transformer secondary X0 using 250.66 & going to the building steel. If there should be a ground fault it will have a low impedance path back to the transformer secondary, so the OCPD can operate.
Otherwise a ground detector will be required 250.21(B).
Otherwise a ground detector will be required 250.21(B).
- Location
- Illinois
- Occupation
- retired electrician
The requirements for a grounding electrode for an ungrounded SDS are identical to those for a grounded SDS. The only code difference between the grounding and bonding rules for grounded and ungrounded systems is the that there is no main or system bonding jumper for an ungrounded system.
tryinghard
Senior Member
- Location
- California
And it's just weird to have an ungrounded wye, possible but weird. Keep in mind the system type is denoted and is such buy the earth connection of a winding so an ungrounded system does not have a winding connection to earth but the frame is connected to earth. See 250.4(B) an ungrounded systems fault path is all the more critical one short will not operate OCP the second will and this is it's intended purpose.
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