ground fault monitor lights

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quinn77

quinn77

Senior Member
Location
Lake Jackson, TX USA
I'm trying to find a drawing of something I came across the other day.
480 volt ungrounded delta service. above MDP is a series of keyless fixtures configured with three columns( three phases ), 2 lights in each column. if there is a ground fault, one light comes on respectively. if there is a phase loss, both lights come on, respectively. this is what I was told by one of the shop hands, so I DID NOT SEE THIS FOR MYSELF. curious if anyone has seen this and how i could obtain a drawing, or better yet an explanation i might comprehend enough to make a drawing myself.
 
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jim dungar

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Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Basically,
The two 240V light bulbs, in series, are connected in a wye configuration, with the center point grounded. A test button, with a closed contact may be wired in series with the center point.

During normal conditions there will be 277V across that set of bulbs (138.5V across each bulb) causing them to glow dimly.

During a phase-ground fault, the voltage across that set of lights becomes 0V causing them to go 'out'. The voltage across the other lights rises to 480V (240V across each bulb) causing them to glow at full brilliance.

When the 'test' button is opened, the reference to ground is lost, causing all of the bulbs to again glow dimly. If there is no change, then you have a burned out bulb.
 
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powerplay

Senior Member
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In front of a computer.
On an ungrounded delta, would there be 277 volts across to ground? I thought there was no way to measure to ground on an ungrounded system, and that it requires ground fault lights on new installations.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
powerplay said:
On an ungrounded delta, would there be 277 volts across to ground? I thought there was no way to measure to ground on an ungrounded system, and that it requires ground fault lights on new installations.
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You are correct, the voltage to ground is not fixed.
However the 'light style' detector works at 277V because of the wye configuration of the lights, not because of any particular voltage to ground.
 
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