Eugene Heil
New member
Situation #1. A factory has 480 VAC but the machine requires 208 VAC (or vice versa). The electrician installs a 3-phase transformer in the room between the service disconnect point and the machine input.
Question) How do you ground the transformer? My guess is that the transformer chassis is bonded to the earth ground wire coming from the upstream service. The transformer secondary (and the neutral going down stream) are bonded to the nearest water pipe (up to five feet), metal building member, or electrode in the ground.
Situation #2. A machine has 208 VAC (no neutral) but internally has 120 VAC to a convenience outlet for instruments. The construction can be either A) one leg providing 120 VAC referenced to ground, like a clothes dryer, or B) a stepdown transformer which in effect makes the machine chassis a "separately derived system". The machine convenience outlet has the neutral pin bonded to the machine chassis. The machine chassis is bonded to an earth ground wire going back to the service. Instruments plugged into the outlet are typically metal box and UL listed. The machine is older than the relatively recent requirement in NFPA 79 which requires these outlets to be GFCI. The European equivalent of NFPA 79 (EN 60204) does not require these outlets to be GFCI. What would you require? Options A,B,C will end up with current on the chassis just like the first panel (service entrance) of a building. Would it be ok to:
A) Bond the machine chassis to the earth ground wire coming from the service.
B) Bond the outlet neutral to the machinery chassis ground.
C) Bond the outlet neutral to the machinery ground if the outlets are converted to GFCI.
D) Pull in a neutral from the service to take care of the outlets.
E) Pull in a separate 120 VAC service and eliminate the internal stepdown transformer.
Question) How do you ground the transformer? My guess is that the transformer chassis is bonded to the earth ground wire coming from the upstream service. The transformer secondary (and the neutral going down stream) are bonded to the nearest water pipe (up to five feet), metal building member, or electrode in the ground.
Situation #2. A machine has 208 VAC (no neutral) but internally has 120 VAC to a convenience outlet for instruments. The construction can be either A) one leg providing 120 VAC referenced to ground, like a clothes dryer, or B) a stepdown transformer which in effect makes the machine chassis a "separately derived system". The machine convenience outlet has the neutral pin bonded to the machine chassis. The machine chassis is bonded to an earth ground wire going back to the service. Instruments plugged into the outlet are typically metal box and UL listed. The machine is older than the relatively recent requirement in NFPA 79 which requires these outlets to be GFCI. The European equivalent of NFPA 79 (EN 60204) does not require these outlets to be GFCI. What would you require? Options A,B,C will end up with current on the chassis just like the first panel (service entrance) of a building. Would it be ok to:
A) Bond the machine chassis to the earth ground wire coming from the service.
B) Bond the outlet neutral to the machinery chassis ground.
C) Bond the outlet neutral to the machinery ground if the outlets are converted to GFCI.
D) Pull in a neutral from the service to take care of the outlets.
E) Pull in a separate 120 VAC service and eliminate the internal stepdown transformer.
