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.

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.

-Hal

[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.

No, I strongly suggest you use something like this to provide the ground. http://www.ditekcorp.com/productDetails.asp?idProduct=DTK-3GTPX

-Hal

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.

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.

-Hal

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.)

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.

-Hal

110.3 still requires you to follow manufacturer's instructions

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.

The Avaya Legend/Magix supports over 200 extensions and uses the same 8/32 green ground screw.

110.3 still requires you to follow manufacturer's instructions

Yeah, some manufacturers will want you to ground the system to a lone ground rod that is not bonded to the service ground. Would you follow that? When I followed Avaya's instructions on how to ground their systems two were damaged beyond repair because of it.

No thanks, I really don't give a RA what the manufacturer says because in this case I really think I know better than they do!

-Hal

You have a point on that too, when the system is damaged who are you going to buy another system from ??

"Yeah, some manufacturers will want you to ground the system to a lone ground rod that is not bonded to the service ground. Would you follow that?"

Is that the manufacturer or the installer?
There are a lot of field people who don't know their ass from their elbow. If it was the manufacturer's instructions I would report the problem to U/L (or the lab that listed it)

The manufacturer, the installer is just following the directions.

Let me ask you this- have you ever seen any other equipment that already has an equipment ground that also requires another ground back to a completely different point in the electrical system?

While in many instances that doesn't present a problem it's never a good idea. You are creating parallel ground paths which can cause current to flow through your equipment.

When we do sound systems we ground everything- the rack, conduit, sat dish and antennas directly to one point at the service ground. We will install an IG receptacle (if NM cable and plastic boxes are not permitted) and land the IG at the same point as the other grounds. No ground loops, no hum.

Mkoloj, if you really have your heart set on doing this according to the manufacturer's instructions, install an IG receptacle for the system (again, if you can't use NM and a plastic box) on a dedicated circuit and take the IG from the same point as your #6. That should make everybody happy.

-Hal



[This message has been edited by hbiss (edited 04-15-2005).]

To clarify, the inter-building transient I was describing (2.5KV, 2 x 10 uS, 500A per lead) is a transient level that can, per Bellcore studies, be expected on lines that are already "protected" by 3-mil carbon blocks. Apparently this is what they found to be the potential "let-through" that occurs often enough that its deemed equipment shall survive at this level. Telco equipment does not have to survive the 2 x 10 uS 5KV waveform.

I did the 2 x 10us 1.5KV 100A intra-building (indoor wiring) tests today on one 12ft length of #10 and also two #10s in parallel. There is hardly any difference between the two - both result in an appr 1 x 3 uS waveform with 500 volt peak at the far end of the ground wire. I also did this with a 1/2 inch by 12ft braid (that is labeled as being ~10AWG equivalent) and found the peak voltage to be 400V - a little better than with normal stranded wire.

I wonder if a 400v 1uS x 3uS waveform can be felt...na, not gonna do it! (for comparison purposes, ESD testing is done with waveforms lasting about 60 nS, but done at levels up to 15KV for most equipment)

To clarify, the inter-building transient I was describing (2.5KV, 2 x 10 uS, 500A per lead) is a transient level that can, per Bellcore studies, be expected on lines that are already "protected" by 3-mil carbon blocks.

Right. That is the primary protector required by the NEC. It's why we install solid state secondary protectors after those TELCO primary protectors. Our secondary protectors have nS response time asa well as sneak current protection and will protect the system pretty well.

I've seen just about everything electronic destroyed in a building because of a lightning near hit. In one extreme instance the covers of our 5 pin protectors were actually blown across the room when they self destructed but our telephone system was always the lone survivor. They did their job.

-Hal

[This message has been edited by hbiss (edited 04-15-2005).]

I am probable too late to add anything here, but what the heck. Do these PBX/key systems have a DC power option? If so then the external ground connection is likely to be for a DC frame ground.

If you were to connect the supplemental ground using AC power sourced from any point other than where the EGC is terminated, you are creating an in/out path (ground loop)for common mode current including lightning to flow through the equipment.

Yes this system can be run on DC so I take it a product like this : http://www.ditekcorp.com/productDetails.asp?idProduct=DTK-3GTPX
Would just be an easy way to put my system at risk by effectively creating a ground loop ??

Mkoloj, no I wouldn’t say that. I am a CO Power Protection Engineer with a phone company and the switches I work with are 100K lines. We use a single point isolated ground method to protect all our switching and transmission equipment. This is standard method used by all telephone companies to protect the equipment. The same method can be incorporated into AC powered PBX and key systems. In fact I was involved in the writing of IEEE Emerald book, chapter 9 which describes the methods and how to employ them. If you are interested read on I will try to be brief and describe it.

Creating what is called a “surge and ground reference equalizer” can use the same technique. In it’s simplest form is what you can use on say your PC by buying a “Point of Use” TVSS device with built in I/O protectors for CATV modem cable, telephone line, and Ethernet. I am sure you have seen them in the store and on e the web. The trick is to buy a quality device rather than the least expensive.

Obviously these type of devices are too small for a PBX/key system as they do not have enough ports to accommodate all the I/O lines needed. But you can sure build one using components and building material.

You start by mounting a ½ or ¾ inch sheet plywood on a wall in a strategic location like between a steel building column and AC service panel. The plywood board will be used to mount cable distribution blocks, TVSS device for PBX power, secondary line surge protectors, PBX (if small), etc. After the plywood is mounted you then have a piece of sheet metal fabricated to cover the plywood exterior surface and secure it to the plywood with contact cement or screws.

Once the plywood and sheet metal work is completed, you bond the sheet metal to any and all nearby grounds available like building steel, ground ring, water pipes, and nearby AC panel ground busses.

Purchase a quality TVSS device with a metallic enclosure, then mount and bond it to the plywood (I recommend using a very short bonding jumper from the case to the sheet metal rather than relying on case contact alone).

Install secondary cable protectors employing solid state devices like avalanche diodes or zener diodes on the plywood then bond the ground terminal directly to the sheet metal using a short as possible bonding jumper. Do not confuse these protectors with the primary protectors provided by the Telco at the point of entrance of the building.

Finish by adding any other equipment like 66-blocks, binding post, D-rings, etc to facilitate cable distribution. If you install the PBX/key system to the board, take the necessary steps to keep the equipment metal case components electrically isolated from the sheet metal. If the PBX is floor mounted also take steps to isolate from any pipes, raceways, or contact with the floor by isolation bushings and pads.

If done correctly, it will be next to impossible for the PBX to have any type of fault current run through it.