Let me set the scene...I am an electrical engineer in Georgia. The installation in question was a 277 volt wall mounted fluorescent vanity fixture in a college classroom building. The ballast is attached to the fixture housing. The manufacturer supplier back plate is mounted to the flush mount metal octagon box. The equipment grounding conductor is bonded to the box and to the manufacturer back plate. The manufacturer does NOT recommend installing a bonding conductor between the metal back plate and the metallic fixture housing. When the fixture is removed from the back plate there is only the phase conductor and neutral from the junction box to the ballast.
Our concern was that when the fixture was energized and not connected to the back plate there was a voltage in excess of 130 VAC measured between the metal fixture housing and the metal faucet at the sink. The building metal water piping is part of the building grounding electrode system.
This is from the manufacturer:
"When they (electrical contractor) break the earth ground connection from the dead metal parts of the fixture, the fixture parts are no longer having their potential measured against the same standard earth ground. While the meter shows a ~ 130v potential, the fixture dead metal parts are not energized from the source power. The meter is indicating a difference in potential between the dead metal parts and earth ground. In fact if they completely remove power from the fixture and measure the potential of the fixture to earth ground they will still see some voltage.
When the connection of the dead metal parts to earth ground is established, the potential difference between the two items is reduced to essentially zero.
There are a lot of contributing factors that drive the potential difference that will be exhibited between floating parts and earth ground. They can check this with other objects and see a voltage reading when the item is not connected to earth ground.
In the best interest of safety, electrical fixtures should not be installed or removed when power is present. We would recommend they turn off the power to the junction box when they are installing a fixture. If they are going to install and remove these with the power present and they want to eliminate this potential between the dead metal parts and earth ground, a ground lead could be added from the backplate to the fixture to maintain the earth ground connection when the parts are physically separated.
The fixtures is safe and certified to be so. Our instructions indicate to remove power before installing or removing the fixtures which is good practice. The voltage they are reading is not because the fixture is energized from the power source, it is due to the fact that the fixture is floating and when compared to earth ground will show a difference in potential."
This seems to be a good argument but I still can 't determine why there would be no bonding strap or conductor between the back plate and the fixture housing. Any comments?
Our concern was that when the fixture was energized and not connected to the back plate there was a voltage in excess of 130 VAC measured between the metal fixture housing and the metal faucet at the sink. The building metal water piping is part of the building grounding electrode system.
This is from the manufacturer:
"When they (electrical contractor) break the earth ground connection from the dead metal parts of the fixture, the fixture parts are no longer having their potential measured against the same standard earth ground. While the meter shows a ~ 130v potential, the fixture dead metal parts are not energized from the source power. The meter is indicating a difference in potential between the dead metal parts and earth ground. In fact if they completely remove power from the fixture and measure the potential of the fixture to earth ground they will still see some voltage.
When the connection of the dead metal parts to earth ground is established, the potential difference between the two items is reduced to essentially zero.
There are a lot of contributing factors that drive the potential difference that will be exhibited between floating parts and earth ground. They can check this with other objects and see a voltage reading when the item is not connected to earth ground.
In the best interest of safety, electrical fixtures should not be installed or removed when power is present. We would recommend they turn off the power to the junction box when they are installing a fixture. If they are going to install and remove these with the power present and they want to eliminate this potential between the dead metal parts and earth ground, a ground lead could be added from the backplate to the fixture to maintain the earth ground connection when the parts are physically separated.
The fixtures is safe and certified to be so. Our instructions indicate to remove power before installing or removing the fixtures which is good practice. The voltage they are reading is not because the fixture is energized from the power source, it is due to the fact that the fixture is floating and when compared to earth ground will show a difference in potential."
This seems to be a good argument but I still can 't determine why there would be no bonding strap or conductor between the back plate and the fixture housing. Any comments?
