Grounding Requirements 2

[JTProE]

Does a 240V delta load from a transformer 480/240 3W transformer need to be grounded? I am looking for code references if possible (I can find wye references, but not for delta secondaries).

Thanks.

 

[itsmoked]

If you mean safety grounded? Yes! How else can a ground fault be detected so the load can be disconnected. And how else do you protect the people when a winding shorts to the motor's case?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JensenDrive]

Would an NEC expert comment: Isn't there some short blurb giving allowance for engineered systems where ground faults are detected by other schemes than overcurrent protection? I looked for it but could not find it.

 

[dpc]

Are you talking about the system ground or the equipment grounding? Even equipment connected to an ungrounded system must be grounded, as itsmoked says.

So by "480/240" you mean a 480 V primary and a 240 V secondary?

 

[JTProE]

The transformer is 480/240 delta/delta. I am asking about equipment ground.

What I am hearing (reading) is that the equipment grounds for motors and other equipment should be connected back to the main system ground....

Correct me if I am wrong....else, thanks for your help.

 

[dpc]

So you would write it as 480-240V, not 480/240V.

The short answer is yes. The long answer is to read NEC Article 250 Section VI on Equipment Grounding conductors. There are some exceptions.

 

[waross]

Equipment grounding means connecting exposed metal parts of electrical equipment to ground. There are very few exceptions for very special cases.
System grounding means connecting one of the supply conductors to ground. A three wire 240 volt delta system may be run ungrounded. However I believe that ground fault detection is required.
respectfully

 

[nchrista]

The nature of a delta is such that there is no neutral to be grounded, I believe it's referred to as a floating delta (though somewhat reduntantly). The only way to ground a delta system is either to break open the delta (thus an open delta) and ground one of the legs or to leave the delta intact and pull 2 phases to ground (though I don't know why you'd want to do that). Somebody correct me if I'm wrong as I'm a bit rusty on this one.

 

[rcw retired EE]

nchrista- A delta winding can be grounded by connecting any one (and only one) of the three phases to ground. Phase-to-phase voltage will be impressed on the insulation systems of the other two phases, but it is better than letting the system float. The delta does not need to be broken.

Many transformers with 240 V delta windings have one winding with a center tap that can be grounded to provide 120V phase to ground on two phases, with the third leg (wild leg) at 208V phase-ground. This would be the best way to ground this application.

 

[JensenDrive]

If you have an ungrounded delta system, a phase to ground fault causes no fault current. Safety grounding of the motor or whatever does not prevent the phase to ground fault to exist indefinitely. So some alternate ground fault detection system is needed. What section of the NEC discusses the need for and design of ground fault detection for this condition?