Ungrounded secondary in production machinery.

[jerrydesaulniers]

I'm work for a machine manufacturer in Canada and currently we are building a hot plate welding machine that will be used in the production of Plastic Auto Parts in the U.S.. I'm troubled by the way we are wiring the 480/240 step down transformer used to power the heating circuits. First I will supply some of the details. The machine is fed by 480 vac three phase and the main disconnect is fused at 150 amps. There are three servos powered by this 480 vac three phase with about 20 amps of load total.Next there is a 480 to 24vdc power supply and finally the circuit in question is the 38.5 kva 480 to 240 single phase(yes single phase) transformer used to power the heating circuits. The secondary of this transformer does not have one side grounded. Instead they have decided to fuse both sided of the secondary and have fused each individual heating circuit with two pole fuses. I brought this practice to question but I'm the new kid on the block. Can anyone guide me to a code violation in either Ontario Code or the US code. Any other feedback would greatly be appreciated. Thank You for your expertise in advance.

 

[GoldDigger]

I'm work for a machine manufacturer in Canada and currently we are building a hot plate welding machine that will be used in the production of Plastic Auto Parts in the U.S.. I'm troubled by the way we are wiring the 480/240 step down transformer used to power the heating circuits. First I will supply some of the details. The machine is fed by 480 vac three phase and the main disconnect is fused at 150 amps. There are three servos powered by this 480 vac three phase with about 20 amps of load total.Next there is a 480 to 24vdc power supply and finally the circuit in question is the 38.5 kva 480 to 240 single phase(yes single phase) transformer used to power the heating circuits. The secondary of this transformer does not have one side grounded. Instead they have decided to fuse both sided of the secondary and have fused each individual heating circuit with two pole fuses. I brought this practice to question but I'm the new kid on the block. Can anyone guide me to a code violation in either Ontario Code or the US code. Any other feedback would greatly be appreciated. Thank You for your expertise in advance.

In the US, the 2011 NEC states in Article 250.21(A):

General. The following ac systems of 50 volts to 1000 volts shall be permitted to be grounded but shall not be required to be grounded: (1) Electrical systems used exclusively to supply industrial electric furnaces for melting, refining, tempering, and the like

However an ungrounded system must include a ground detector (not a ground fault interrupter) for safety, as specified in (B) of the same Article.

 

[Smart $]

In the US, the 2011 NEC states in Article 250.21(A):
However an ungrounded system should include a ground detector (not a ground fault interrupter) for safety.

FWIW, the NEC doesn't apply to manufactured machines other than the supply or other field wiring.

 

[GoldDigger]

FWIW, the NEC doesn't apply to manufactured machines other than the supply or other field wiring.

I missed the part where the OP said they were manufacturing the machines.
To be used in the US, they may have to have US standard approval by an NRTL (which could be CSA), and the whole question would then come down to the applicable UL standards.

 

[jerrydesaulniers]

Thanks Gold Digger

Thanks Gold Digger

I appreciate the reference to the code and I had found that myself earlier. But I was hoping for some other articles in the code that I had not already found. Do you know what the dangers are of an ungrounded secondary. I measured the voltage to earth and it was around 150 volts of potential but I have no way of knowing if a short to earth could actually safely take out the fuse. The problem is that I'm the new guy on the job and I want the ammunition to make them do the right thing. Any logical discussion might help my case.

 

[GoldDigger]

The advantage of an ungrounded system is that the first fault to ground will not trip any fuse or breaker because no fault current can flow.
If that first fault is detected, you can continue to operate the equipment until it is safe or convenient to shut down for repair.
It is the second ground fault (or a line to line fault) that will blow the fuses.
Studies have shown that the first fault from insulation failure is more likely to be a ground fault than a line to line fault.

Sent from my XT1080 using Tapatalk

 

[texie]

I'm work for a machine manufacturer in Canada and currently we are building a hot plate welding machine that will be used in the production of Plastic Auto Parts in the U.S.. I'm troubled by the way we are wiring the 480/240 step down transformer used to power the heating circuits. First I will supply some of the details. The machine is fed by 480 vac three phase and the main disconnect is fused at 150 amps. There are three servos powered by this 480 vac three phase with about 20 amps of load total.Next there is a 480 to 24vdc power supply and finally the circuit in question is the 38.5 kva 480 to 240 single phase(yes single phase) transformer used to power the heating circuits. The secondary of this transformer does not have one side grounded. Instead they have decided to fuse both sided of the secondary and have fused each individual heating circuit with two pole fuses. I brought this practice to question but I'm the new kid on the block. Can anyone guide me to a code violation in either Ontario Code or the US code. Any other feedback would greatly be appreciated. Thank You for your expertise in advance.

I think that this is a good example is why many AHJ's require testing and approval by a NRTL of all equipment such as this. It would be up to the NRTL to determine if this can be operated ungrounded based on the standard it is being tested to.

 

[jerrydesaulniers]

A gentle spirit solved the problem

A gentle spirit solved the problem

I went to the person in charge and kindly asked him to reconsider the ungrounded secondary and my input was well received. I'm glad that I'm not involved in a dangerous project any more.