# Technical description of GFI's

**URL:** <https://boards.straightdope.com/t/technical-description-of-gfis/26488>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [July 22, 2000, 10:11pm UTC](https://boards.straightdope.com/t/technical-description-of-gfis/26488 "2000-07-22T22:11:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![cheshirecat](https://avatars.discourse-cdn.com/v4/letter/c/34f0e0/32.png) [@cheshirecat](https://boards.straightdope.com/u/cheshirecat)\
**Post date:** [July 22, 2000, 10:11pm UTC](https://boards.straightdope.com/t/technical-description-of-gfis/26488/1 "2000-07-22T22:11:25Z")

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Cecil says that “If volts in exceeds volts out by more than a certain percentage, that means some of the current is taking a detour,…”

He’s right about these things comparing currents, but they are in “amps”, NOT “volts”. I doubt if any GFIs measure volts of any kind!

If we’re supposed to be talking everyday technical matters, (we’re not talking about the more controversial aspects of nuclear physics) let’s at least get the simple basics right.

By the way - I think the “percentage” bit is probably not right either. The thing needs to trip as soon the discrepancy in current exceeds the human danger level, (about 50 milliamps) irrespective of the total current being used by the hairdryer or whatever.

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**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\
**Post date:** [July 23, 2000, 2:14pm UTC](https://boards.straightdope.com/t/technical-description-of-gfis/26488/2 "2000-07-23T14:14:48Z")

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Welcome to the SDMB, and thank you for posting your comment.  
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Cecil’s column can be found on-line at this link:  
[What does a ground fault interrupter do? (12-Aug-1994 )](http://www.straightdope.com/classics/a940812b.html)

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**Author:** ![Arjuna34](https://avatars.discourse-cdn.com/v4/letter/a/eb8c5e/32.png) [@Arjuna34](https://boards.straightdope.com/u/Arjuna34)\
**Post date:** [July 23, 2000, 6:15pm UTC](https://boards.straightdope.com/t/technical-description-of-gfis/26488/3 "2000-07-23T18:15:28Z")

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> [@](#):
>
> \*Originally posted by cheshirecat \*  
> \*\*  
> By the way - I think the “percentage” bit is probably not right either. The thing needs to trip as soon the discrepancy in current exceeds the human danger level, (about 50 milliamps) irrespective of the total current being used by the hairdryer or whatever. \*\*

You’re right about the “percentage”- the GFI is set to trip at an absolute current level. It’s usually around 3 or 4 milliamps, though, not 50.

Arjuna34