Alarm & Grounding

[techhouse]

Greetings,

A current alarm panel installation has the main control board, which is inside the provided can, as well as two expansion modules that will be located adjacent to this main control board and can. All three devices have ground terminals. There are no copper pipes, etc. to which these three devices can be joined and subsequently grounded (a 6 AWG ground wire will then be brought outside and a new ground rod will be installed). So, my question is, what would be the proper way to ground these three devices? I thank you for your assistance.

Kevin

 

[nvcape]

Re: Alarm & Grounding

If I understand what you are doing, you need to follow NEC 250.4.A.2 and 4. You still need an equipment ground conductor back to the power panel. "The earth shall not be used as the sole equipment grounding conductor..."

 

[techhouse]

Re: Alarm & Grounding

Thank you for your reply. Yes, you do understand me correctly. I guess my question should have been: do I run 14 AWG ground wires from all three device terminal screws and then join them at the power panel, and then run a 6 AWG to a new ground rod (supplementary), which will then be bonded to the existing rod? Is this correct?

Kevin

 

[dereckbc]

Re: Alarm & Grounding

As nvcape said you are required to run an EGC with the branch circuit conductors supplying the alarm panel with power to provide protection for the AC circuit. The ground terminal in which you are referring to is a supplemental ground connection.

These supplemental ground terminals can be bonded to the GEC conductor if building steel, water pipe, or no other electrode is available. But the point here is all the electrodes must be bonded together to form a common electrode. You cannot just drive a rod and leave it isolated from the rest of the electrode system.

EDIT: In closer review of your second post, you could bond all three terminals together, then run a 6 AWG back to the ground rod that the AC service uses. But why? The impedance difference between the 14 AWG EGC and the supplementary 6 AWG GEC is of no effect. Are there any lightning protectors serving outside cables?

[ October 30, 2003, 01:21 PM: Message edited by: dereckbc ]

 

[hbiss]

Re: Alarm & Grounding

These supplemental ground terminals can be bonded to the GEC conductor if building steel, water pipe, or no other electrode is available.

I would be wary of using building steel or a water pipe however. Case in point: We regularly install and service small to medium sized telephone systems from a well known manufacturer. Some time ago we got a call from a law firm that we had installed at least two years prior. The complaint was that the entire system was down.

When I arrived I determined that the processor module was completely dead. This module has the line cord for the system as well as a supplemental ground screw. The manufacturer also suggests that this screw be connected to building steel or a cold water pipe. Since this was on the third floor of a large office building, when this was installed I put a clamp on an "I" beam above the dropped ceiling to provide the supplemental ground. There is usually nothing else (economically) available in these offices.

I replace the module with a new one ($750.00), reconnected the supplemental ground wire and plugged the line cord in. Everything worked fine. Two days later I again get a call that the system is down.

Again the processor module is dead. I replace the module (and start to think about power surges) but this time I just happen to notice that after a few minutes the line cord is warm! It's an 18ga three wire cord but this only draws about 75 watts. For some reason (luck I guess)I disconnected the supplemental ground wire from the screw before I pulled the plug and got an arc! Got the meter out and there was 50 volts between the "I" beam and the ground on the receptacle. Why now after 2+ years? Customer said that there was some kind of electrical work being done for the building owner at night apparently by some "fly by nighters".

Now, the current flow between the grounds wouldn't be so bad if the morons at this "well known company" had hardwired with a piece of wire the supplemental ground screw within the module to the green ground wire of the line cord. At worse the wire would have burned up. Instead they used a circuit board trace to connect the two and it had a bunch of other connections to it along the way. Once the trace burned in half the voltage differential destroyed components on the board and that was the end.

Moral of the story is we no longer provide a supplemental ground unless we can connect it to the same point (and within close proximity) as the receptacle ground.

 

[brian john]

Re: Alarm & Grounding

Why do you think you need a Ground Electrode for these panels?

 

[hbiss]

Re: Alarm & Grounding

...do I run 14 AWG ground wires from all three device terminal screws and then join them at the power panel, and then run a 6 AWG to a new ground rod (supplementary), which will then be bonded to the existing rod?

Yes, why would you want to drive another ground rod? How far from the panel are these alarm devices?