What is the proper way to ground a phone system? I know I need a #6 awg. Should it go to the nearest vertical building steel, back to the ground bus in the electrical panel. The installation is a small office building with about 20 users and 1 electrical panel about 50' from where my system will be. Where I am we have our in-house electricians doing the work and I need to make sure it is done correctly as they are known to take a below standard and easy way out on occasion and I don't want it to compromise my phone's.
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By phone system I assume you mean the KSU or PBX equipment itself. We always install secondary CO line protectors usually at that location also. We normally run something like a #6 or #4 from a ground bus on the telephone backboard back to (ideally) the service ground. In an office building this may not always be possible and a sub-panel ground or building steel will have to suffice.
I would be careful about grounding the KSU or PBX equipment. Any such ground (lug or screw on the equipment) is usually supplemental to the line cord ground. I have had instances where the KSU was destroyed because a ground loop caused a great deal of current to flow from the building steel through the equipment and out to the line cord ground (the line cord was actually hot to the touch!) This happened one day after being in service for more than two years. We have no idea what caused the large voltage differential to occur between the building steel where the equipment ground originated and the sub-panel ground where the receptacle ground originated.
After that (which cost the customer thousands) we either don't connect the supplemental KSU ground or connect it to the exact same point that the line cord is grounded to which is the receptacle ground.
[This message has been edited by hbiss (edited 04-12-2005).]
It is PBX equiptment (Nortel Opt 11 cabinet). This is in an old retrofitted building in NYC so if the building steel is to be used it would be good to know that it is in fact a low-impedance path to ground. I think with the construction of this building I will have them bring it back to the service to be safe. If I have the condition you described happen we will be out a little more than $2k with this system. Thanks for your insight Hal.
Re: Grounding Phone Systems #159986 04/13/0512:59 PM04/13/0512:59 PM
I see that the product you linked is just plugged into an outlet.Does it just put the supplemental ground in parallel with the ground in the line cord? Nortel specifies a #6 conductor to be used. Is that product going to present a bottleneck if it is indeed just giving you the ground out of the outlet and putting it on a terminal on the front of the product? As the outlet ground wire is a #12 or #14 and undersized in comparison to the #6 connected to the lug on the cabinet.
Re: Grounding Phone Systems #159988 04/13/0507:20 PM04/13/0507:20 PM
Have to remember that some manufacturers live in a fantasy land when it comes to grounding. I can't imagine why a #6 ground would be needed on something like this. This supplemental ground is there because the system is connected to premises wiring and if the line cord is disconnected nothing would be grounded. That could be a problem in the event a run of wire going to an extension was to become energized.
But keep in mind that the premises wiring is only 24ga, the line cord is probably 18 or 16ga plugged into at most a 20A circuit. The CO lines should be protected ahead of the system. Why would a #6 be needed? We run a #10 to the ground binding post on that Ditek protector and that's overkill.
The ground wire on a phone system is there to keep the chassis and telco leads at a reasonable voltage under various fault conditions. For faults to 60 Hz 20 amp circuits, a #10 wire will have no problem clearing a breaker, and additionally the chassis will not assume a hazardous voltage during the event. The 24AWG or 26AWG wire will light up like a fuse. Lightning transients are a different story. A year or so ago I did some testing on various grounding and cable bundling schemes. What I measured then, and verified just minutes ago, is when you take the level of lightning surge that is within limits where high quality telco equipment is not expected to fail, and where #26 AWG wire is not damaged, the chassis will take a significant jump in voltage. Suppose your equipment has just one four-wire interface that clamps overvoltage to ground, and the lightning surge present is 2.5 KV 500A per lead (per Telcordia GR-1089) with a 2 x 10 uS waveshape. The equipment will route the 2 x 10 uS 2000A (peak) surge to ground. The voltage waveform measured across 12 feet of #10 stranded wire to ground is approximately 1200 volts peak and about 1uS x 3uS, dropping to zero at 5 uS from start of the surge and then going negative to about 350 volts (from the inductance of the wire). This is what would be seen on the "grounded" chassis of the telco equipment if it has outside plant wiring and a line gets hit with what is considered a survivable transient (survivable to your equipment that is). If the system has internal phone lines that clamp to this ground, then they will also see the sudden bump.
For PBX systems that are only grounding internal wiring, and not also providing ground for outside plant circuits, the applicable surge level is significantly less (1500v 100A per lead). I have not ran this test with these lesser surges.
The problem with the #10 wire is the inductance. It is better to use a wide braid, bus bar, shorter wire, or bigger wire (#6), but there will still be some movement of ground under circumstances that do not damage normal telco wiring.
(I was rather surprised at these results back when I first did this testing. I'm now more careful about leaning against "grounded" metal when doing lightning tests.)
Re: Grounding Phone Systems #159990 04/14/0508:14 PM04/14/0508:14 PM
For PBX systems that are only grounding internal wiring, and not also providing ground for outside plant circuits...
A PBX or KSU sould ONLY be grounding the inside (premises) wiring and should not be expected to provide any protection for CO lines or extension wiring that goes beyond the building. Any ground connected to the system chassis would not need to account for this. The outside plant and any off premises extensions (OPX's) are required by the NEC to have protectors where the conductors enter or exit the building.
We go a step further and provide secondary listed protectors for each CO line and OPX that clamp at 235V and have overcurrent protection that will open the pair in the event of a sustained current event.
I should point out that it is quite common for some manufacturers to simply bolt a good size pool lug on the cabinet for a ground. Your Nortel system with only 20 extensions "requires" a #6 ground. We sell Avaya systems that can accommodate up to 48 extensions and all they provide is a green 8/32 ground screw and no wire size requirement.
We currently will only have 20 users, that gives us 20 sets, 3 fax machines and 3 credit card swipe machines. The system is capable of having 160 extensions when outfitted with the maximum # of line cards in the cabinet. So it is a bit larger than the Avaya system Hbiss is talking about.