isolation transformers

[electricalperson]

recently ive worked on an isolation transformer at work. when i put my meter on the secondary i had 61 volts from each wire (hot and neutral) to ground and 120v L to N. the isolation transformer was feeding a faraday cage for plugs and a few lights inside. i called my boss and told him im getting this voltage and he said this is normal. the neutral wasnt grounded inside the transformer and i was a little confused..is this normal?

 

[mdshunk]

Actually, you didn't really have a neutral in that situation. You had two hots, each with 61 volts to ground, and 120 between them. What would have been the neutral (the center tap) was likely just bonded to the chassis.

 

[electricalperson]

so how do i create a real neutral? directly bonding the neutral to ground?

 

[mgd]

your high input impedance meter is "grounding" the secondary. you momentarily created a voltage divider. primary/secondary coils in series with meter

 

[electricalperson]

i used a wiggy at first and i was getting voltage at each conductor then used a DMM and found the 61 volts

 

[mdshunk]

so how do i create a real neutral? directly bonding the neutral to ground?

Whaaa? For what purpose? The "neutral" is already bonded to ground in a store-bought isolation transformer. You don't need it anyhow, unless you plan on using 60 volts for some purpose. It's just your EGC for all intents and purposes.

 

[electricalperson]

thanks for your help i appreciate it

 

[don_resqcapt19]

In most applications, 250.20(B)(1) would require one of the transformer secondary conductors to be connected to a grounding electrode making it a grounded conductor. A true isolated system is rarely permitted by the NEC. In this case in addition to the connection to the grounding electrode, there would also be a connection to the primary EGC.
Don

 

[hardworkingstiff]

.... A true isolated system is rarely permitted by the NEC.

The only one I've come across is in the O.R. of a hospital. The purpose is to keep someone from getting shocked if a piece of equipment develops a problem while performing surgery (there is no grounding anywhere on the secondary). If a fault is detected, an alarm indicates a problem, the surgery is finished and that room is not used again until the fault is fixed. This was back in the 70's, I assume this is true today.

 

[roger]

Lou, in NC Isolation Panels and Line Isolation Monitors are still a requirement in O.R.'s, ICU's, Triage room's, etc....

Roger

 

[acdcmontana]

protected by a GFI?

protected by a GFI?

For these line-isolation type panels, such as SQD 4800; is the feeder supposed to be protected by a GFI?

 

[roger]

The answer is no to needing GFP for the feeder supplying the transformer of an Isolated Power Panel


Roger