Burned ground wire after loss of 1- phase

[mike k]

Recently our service department responded to a service call after an electrical outage.
The customer lost 1-phase of a 120/208 Y system. The piece of equipment in question that would not work after the lost phase was restored by the power company was a printing press. After our service tech. inspected the wiring we found several problems.
The equipment was feed from a 120/208 volt 200- amp fused disconnect with 150-amp fuses. It then went to a 45 kva 120/208Y (Secondary) to 480 delta (primary) transformer connected in reverse to get 480-Volts. From this transformer it feed another multiple tap transformer that took the 480-volt primary and adjusted it to the machinery factory voltage of 380-volts. The disconnect had 150-amps feeding it from a MDP via EMT conduit, the 1st violation was that the neutral and the grounding conductors were connected together and bonded to the disconnect can. From the disconnect (Secondary side) to the 1st transformer was feed with (3) # 1 wires and (1) #4 wire for the ground, the ground was connected to the can in the disconnect (where the neutral & the ground were bonded) and connected to the transformers XO connection which was also bonded to the can this being the second violation. From this point the 480-voltage and the ground connected to the transformer can (where the XO was bonded) was taken to the 2nd transformer and connected to the can and the (3) phase went to the primary side. The secondary side of the 2nd transformer was connected to the printing press using the same equipment ground that came from the 1st transformer Via the 200-amp disconnect. The ground wire was #4 connected to both transformer cases and bonded to the 1st transformers XO neutral connection. This wire is the problem from the disconnect to the 1st transformer the insulation was completely melted and the conductor was starting to melt. And as for the conduit it also had signs of carrying a load it was black at the connectors and had appeared the it to was starting to melt the heat was very excessive and the plastic bushings were melted to the pipe. Eventually it blew (1) of the 150-amp fuses in the disconnect. The customer had told us this exact same situation occurred before when they lost a phase during a storm. (A couple of years ago)

1) Should the neutral be connected to the 1st transformer for this application?
2) Should it be bonded to the transformer can?
3) If so the conductor that was installed was too small (#4) and should be changed to a #1?
4) What caused this wire to breakdown after loosing 1?phase?
5) After reviewing the code I believe that each of the (2) transformers on the secondary side should be bonded to the building steel form the can?

 

[jim dungar]

Re: Burned ground wire after loss of 1- phase

When a low voltage transformer is connected as a wye-delta the X0 terminal should not be connected to the neutral of the wye system and should never be bonded to ground.

I got a little confused with your description of primary and secondary. Truthfuly, the H connections are for high voltage (not primary) and the X are for the low voltage (not secondary).

There will be discussion on if your reverse connected transformer is "per code". Some manufacturers (i.e. Square D) bless and promote this arrangement, other manufacturers do not.

 

[bob]

Re: Burned ground wire after loss of 1- phase

Mike
If you are not going to use the neutral, the code does not require you to carry it along with the phase conductors. As you already know you must install a EGC to each transformer and ground the frame of the transformer. When you have a phase loss (single phase condition)the neutral current will approach the phase current. The neutral and the EGC in parallel with the conduit became the return path. This can be shown by the fact that the conduit and the EGC showed sings of excessive overload. The phase load can approach twice the normal load. You can eliminate this by installing time delay fuses with a value of about 110% of the full load amps of the motor. Check Bussman and they have a list of fuses for different load currents. What was the FLA of the 380 volt motor?
You might also check into one tranformer with taps that will allow you to go from 208 to 380 by adjusting the taps for the 480 volt secondary side. Another means of protection is the use a relay to energize the motor and install a device that will sence the presence of all 3 phases. If one phase is out this device will trip the relay and de-energize the motor. FLA amps of the motor?

 

[charlie b]

Re: Burned ground wire after loss of 1- phase

1) Should the neutral be connected to the 1st transformer for this application?
No. Since the Wye side of this transformer is serving as the primary, the neutral serves no purpose.

2) Should it be bonded to the transformer can?
Not applicable. See response to item 1.

3) If so the conductor that was installed was too small (#4) and should be changed to a #1?
It can be removed.

4) What caused this wire to breakdown after loosing 1?phase?
I agree with Bob. Since the neutral was connected, the loss of one phase caused the neutral to function as a phase conductor ? it carried full phase current. Then, since it was inappropriately bonded to the EGC, this caused the neutral and the EGC (as a set of parallel conductors) to carry full phase current. This was too much for a #4, and that is the cause of its being damaged.

5) After reviewing the code I believe that each of the (2) transformers on the secondary side should be bonded to the building steel form the can?
I agree.