Neutrals connected in parallel on control transformers.

[electron mover]

While doing loop checks on a new DCS system in a power plant MCC center with the buckets removed; I discovered all the neutrals were connected in parallel from neutral stab to neutral stab and laid down on the ground buss in each section with 14ga wire.

Each control transformer's neutral is grounded where it is mounted in the bucket. The transformers are from 100 to 500 VA and 480V pri. to 120V sec.. The primary's on these transformers could be out of phase and there are over 50 cubicles in this center. It is possible that some of these transformers may share neutrals to some field devices.

This is an older installation from the early 70's and is a GE MCC. I see some risk here should the neutral become lifted off the transformer or become loose and yet have a return path through all the parallel neutrals and through the bonding wire back to the source.
Are my concerns legitimate? Dose anyone see any code violations here?

Thanks,
Electron Mover

 

[Rick Kelly_home]

Control Transformer Neutrals...

Control Transformer Neutrals...

I can not comment on any potential code issue as I know little of the NEC (I am a Canadian inspector).

If this was a Canadian installation, I would expect to see it as you described it.

If a transformer were to loose its return path; there would no longer be a path back for current and that bucket would no longer have a "live" control circuit and would no longer function.

 

[electron mover]

I see your point Rick, but I also see heating and possible arcing should the neutral or grounded conductor become opened and use the equipment ground as a return path because this situation could provide numerous paths. This could continue for a long period of time and also pose some touch potential risk for employees servicing this equipment. Article 250.142 of the 2005 NEC handbook describes these effects of a grounded conductor grounded on the load side of the neutral. Though this is not truly on the load side, the return path through ground under the correct conditions could be some distance from the source.

I know the practice of wiring MCC's like this is done at times, I'm just brain storming as to what could happen in the worst case situation should the ground be used as the sole return path for current which is made very likely by tying all the neutrals together from numerous control transformers.

 

[Rick Kelly_home]

If the neutral lifted off of the transformer (as in the only connection to the transformer was the "hot" side) there would be no return path, even through the "equipment ground". If the neutral point of the transformer no longer had continuity to the neutral bus or to ground, the only way current, in that transformer, could flow, would require a short in the winding of the transformer itself to ground to complete the path to the multiple neutrals you mention.

I have seen this, a shorted winding, happen once or twice in older MCC buckets, they were Cutler Hammer if I remember correctly.

Usually the control fuse would pop. It is true, however, that whether or not the fuse blows depends upon the impedance of the winding after the short to ground.

When the fuse does not blow, the winding will usually run hot till it pops on another turn.

Having said all the above... I have not seen a transformer do this is many a year.

Isn't brain storming fun...

 

[petersonra]

I think all MCCs with control transformers are done this way.

This is no different than any other SDS that uses the EGC to ground the neutral.