ECN Forum Forums General Topics of the Trade Voice - Data - Video Discussion Grounding Phone Systems

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.