Voltage on the Ground

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Ronnie Barham

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I’ve finished a new add on to a church, the fire alarm contractor says there is 1/2 of a volt from my ground to his alarm wire and that his system will not work with this 1/2 volt present. Everything is per the engineering plans and no other problems. Anyone else had this problem?
 
181011-2015 EDT

Ronnie Barham:

What is "ground"? Why does your "ground" have anything to do with the "fire alarm contractor"'s system?

Let us assume that "ground" means an EGC conductor at some undefined point. In the real world there is no reason to expect that the voltage at some random point on an EGC is the same as the EGC bus at the main panel. Ideally this difference would be zero. There are many reasons why it might not be zero. So we need to know to what point on your EGC the "fire alarm contractor" made his measurement. Where, how, and why is the fire alarm system referenced to your EGC?

More information is needed.

.
 
gar said:
181011-2015 EDT

Ronnie Barham:

What is "ground"? Why does your "ground" have anything to do with the "fire alarm contractor"'s system?

Let us assume that "ground" means an EGC conductor at some undefined point. In the real world there is no reason to expect that the voltage at some random point on an EGC is the same as the EGC bus at the main panel. Ideally this difference would be zero. There are many reasons why it might not be zero. So we need to know to what point on your EGC the "fire alarm contractor" made his measurement. Where, how, and why is the fire alarm system referenced to your EGC?

More information is needed.

.
Click to expand...

ptonsparky said:
Why does he think the elevated voltage is your problem? You need more information.
Click to expand...

I believe the OP tried to reply and hit the "report post" button. Here is his reply.

I have a 12/2 MC 20 amp circuit going to his control box, there is a 1/2 volt between the insulated ground wire in the MC to his fire alarm positive wire.
Click to expand...
 
181011-2232 EDT

Still too little information.

Apparently there are three conductors in the MC cable. One is an insulated green wire for the EGC. We assume this cable only supplies the alarm box.

I would do the following:

1. Open the EGC wire at the alarm box.

2. Make a long test lead from the main panel EGC bus to this alarm box location. Can use extension cords, or roll out some wire for this lead. The wire size of the test lead could be almost anything, but #20 or larger would be mechanically good.

3. Using a high impedance meter, 10 megohm input impedance, measure the voltage between the test lead and the MC EGC wire. Should be in the millivolt or less range. Keep the long test lead close to the MC cable if possible. This reduces voltages induced into the one turn loop from stray magnetic fields.

4. Keeping the meter connected as in 3 reconnect the EGC to the alarm box. There should be no appreciable change in the voltage reading. If there is it is caused by the alarm box. What is the current in the EGC?

5. If no voltage change, then there is a problem in the alarm box or its circuit.

Provide information from the above tests.

.
 
... I have a 12/2 MC 20 amp circuit going to his control box, there is a 1/2 volt between the insulated ground wire in the MC to his fire alarm positive wire.
Click to expand...

All that tells us is the voltage is coming from within the panel. You have an alarm panel at the end of a run of 12/2 MC with the ground screw in the alarm panel bonded to the green ground conductor. Unless there is some other ground connection someplace connected to that panel, there is no way that there can be a voltage present between the positive wire and the ground wire unless it's coming from the panel.

Disconnect your green ground and measure for voltage between the screw and the positive wire. Betcha it's still there.

-Hal
 
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