Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


GFCI receptacle question

GFCI receptacle question

GFCI receptacle question

Since the GFCI does not use the equipment grounding conductor in the sensing circuit, GFCIs can protect non-grounding circuits and can replace two-wire (ungrounded) receptacles. However, my specific question deals with an isolation transformer that is protected (receiving power from) a GFCI receptacle.

IF there is leakage on the SECONDARY side of the isolation transformer, then the GFCI protection for the transformer (primary side) is quite useless, correct? It will not trip because it cannot sense any change, correct?

RE: GFCI receptacle question

Yep, that is correct.
You need separate protection on the secondary side also. You usually need to ground that secondary side so it doesn't float to an undefined value.

Keith Cress
kcress - http://www.flaminsystems.com

RE: GFCI receptacle question


What's the "isolation" transformer and how is it wired. Maybe, maybe not. Another question for the ultimate protection answer of "it depends".

RE: GFCI receptacle question

The GFCI compares the current going out with the current returning. If there is a difference the GFCI assumes that the lost current returns via ground and trips.
A ground fault on the secondary of an isolation transformer will not affect the balance of the primary currents.
In large protection schemes there may be unintended interaction or lack of protection with multiple grounds and poor design.
In large systems ground fault protection may be provided by monitoring the current (or lack of current) in the neutral to ground link rather than monitoring the residual current. David and I have both encountered systems where ground fault protection has been compromised by poor design. The improper addition/design of a standby generator may cause ground fault protection issues.
It depends.
With a receptacle on an isolation transformer wired according to code there is not much wiggle room.
If the installation is not to code then it depends.

"Why not the best?"
Jimmy Carter

RE: GFCI receptacle question

The ungrounded secondary of an isolation transformer will not leak until both legs short to ground. If this happens, overcurrent protection will operate. This does not make the GFCI on the primary side useless, since it still protects the primary circuit and the primary side of the transformer. It may also serve to make the AHJ happy.

RE: GFCI receptacle question

I'm not sure how you can say that. There can be leakage on one of the leads only. Point noted that overcurrent will trip if both leads short to ground.

RE: GFCI receptacle question

Unless the secondary is extensive enough to have capacitive coupling to ground, ground is simply not part of the circuit. For current to flow, a path back to the transformer secondary winding is required.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close