Ground Fault ideas

Status
Not open for further replies.
B

brother

Senior Member
Location
"I have learned, in whatsoever STATE I am, therewi
I havent installed one that was set this low. Can you use a current transformer that will be set for/read 5 milliamps across a line and use it to trigger a shunt trip breaker on 277 volt circuit??

You probably asking WHY would i want to do this, well just for brain storming idea on a 277 circuit that needs to protect PEOPLE instead of just the equipment. Als wanted to know if you can set a current transformer for that low.
 
I doubt that you could connect a CT directly to a shunt trip breaker and get any sort of reasonable or controllable sensitivity for this application.

You can purchase components (such as 'ground fault relays') which would serve the function that you describe; measuring the output of a CT and tripping a shunt trip breaker.

You can purchase 'Class A' GFCI systems pre-built for up to 600V: http://www.bender.org/?gclid=CNWqleTGi5YCFQgWiQod4R4dFg

Heck, Leviton almost makes something suitable http://www.levitonproducts.com/catalog/model_8895.htm (this requires an external, UL approved contactor to work, and while it could certainly drive a shunt trip breaker directly, I doubt that such is currently an approved combination to be rated as a GFCI...and the sensor may only be rated for 250V. Since the sensor only cares about current, I presume that the rating is for the insulation.)

-Jon
 
081003-1213 EST

A GFCI device uses a current transformer, and its nominal threshold is 5 MA.

See my web site at
http://beta-a2.com/GFCI.html
and the third photo labeled P5.

Seems all the photos after P5 are P5.

The following thread has considerable information, and sources of, on GFCIs:
http://forums.mikeholt.com/showthread.php?t=102022

Various current measuring devices use a current transformer, and you can measure moderately small currents.

.

.
 
081003-1213 EST

A GFCI device uses a current transformer, and its nominal threshold is 5 MA.

See my web site at
http://beta-a2.com/GFCI.html
and the third photo labeled P5.

Seems all the photos after P5 are P5.

The following thread has considerable information, and sources of, on GFCIs:
http://forums.mikeholt.com/showthread.php?t=102022

Various current measuring devices use a current transformer, and you can measure moderately small currents.

.
 
081003-1238 EST

Can you measure a 5 MA change from 200 A with a current transformer? Maybe not. That is 1 part in 40000.

You need to add detail on how you want to use the current transformer relative to what you want to detect. Is it a different function than a GFCI?

.
 
Yes, please be more specific.

Is this the case that you want certain equipment to shutdown if a person does something accidental?
 
Billy_Bob said:
Yes, please be more specific.

Is this the case that you want certain equipment to shutdown if a person does something accidental?
Click to expand...

basically i didnt think they made a 277 gfci 20 amp breaker. I just wanted the 277 power to be shut off if there was a 5milliamp fault. these are low lights and its very small chance it can happen but if someone was touching the metal of that light i want the power off. And i was just wondering if it was possible to do it that way.
 
A true GFCI (Class A) has a specific time versus current tripping curve. It is unlikely that a stand alone CT feeding a standard shunt trip mechanism would be able to operate within that trip curve.

Yes you can build a GFI device like you describe, but it does not make it a GFCI suitable for "people" protection.
 
081003-2051 EST

brother:

If you took the circuitry of a standard GFCI device, created a current transformer with a greater over the surface and internal voltage breakdown between the power conductors and the current transformer output, connect this to a standard GFCI circuit, used a bigger SCR to operate the trip mechanism, and ran it thru UL, then you would have a suitable device.

If all your metal enclosures of your lamps are properly connected to their EGC, then touching the metal enclosure and earth would not normally cause a shock. However, touching something that is hot and the metal enclosure would cause a shock, or if something produced a high voltage on the EGC, then touching the sheetmetal and earth would cause a shock.

If you had a suitable GFCI on the source of the high voltage relative to earth or a properly connected EGC, then only a short duration shock would occur.

.
 
The trip threshhold for personnel protection is 6 ma . Can you build something that will trip at that low of a value , AND within 25 milliseconds ?
Just asking .
 
The trip time of a GFCI is not required to be that fast.

From one of the other moderators.

don_resqcapt19 said:
It is UL standard #943 and it says: The maximum permitted time to trip in seconds is equal to the quantity (20/fault current in milliamps) raised to the 1.43 power."
That actually permits a 7.2 second maximum trip time. Note that while the standard permits this as a maximum trip time, most devices will trip in a much shorter time.
Click to expand...
 
djohns6 said:
The UL regulates products . What does OSHA and the NEC say about such
a home built installation ?
Click to expand...

I don't know but I am not sure it matters.

The item he wants to GFCI protect is not required by any code section to be GFCI protected.

As long as all the required protections are already in place I do not see any code / UL / OSHA reason he could not go above and beyond.

That said, I would not attempt in my wildest dreams to home build a circuit protection device. I would wire the equipment following the NEC and standard installation procedures and feel good about it.
 
I guess my point is , if you are building a circuit for PERSONNEL protection , you would need to comply with a bazillion regulations and still open yourself up to civil liability .
 
djohns6 said:
I guess my point is , if you are building a circuit for PERSONNEL protection , you would need to comply with a bazillion regulations
Click to expand...

If the device he is building is beyond any code requirements I see no regulations he would have to comply with.

open yourself up to civil liability .
Click to expand...

That I agree with 110%, I would not put myself or the company I work for in that position.
 
benaround said:
Brother,

It might make more sense to change the fixtures to 120v and use standard

GFCI breaker.
Click to expand...


I agree, and thanks for all the responses. I was just curious as to if this would be possible with a 277volt sitation. There are alot of lights. Thats why they use 277 volt. As someone said if i did this, it would be above and beyond what the code requires. That being said, even if this did not work i do NOT see any libility in this 'particular' situation. In others i could see it.
 
iwire said:
5 ma @ 277 is likely lethal.

I suspect you would need to drop to 2 ma for roughly equivalent protection
Click to expand...


Why is it any different than 5 volts? Isn't 5 ma, 5 ma regardless of the voltage? Granted there is a difference in the resistance of these two circuits. A lot more energy is disipated, and more burns, but to the heart is it not the same?
 
Status
Not open for further replies.
Top