Grounding TV station

[fc]

Looking at a job today at a TV station. They are having problems with grounds.
They have a 225kva transformer 480 / 120-208 3 phase that feeds the MDP in the same room. In the MDP they have five - 200amp feeders installed in EMT all 3 phase 4 wire going to panels in there computer room. The computer room has six cable trays over the computers. The GEC for the transformer goes to a copper plate on the wall and from this plate the GEC goes to building steel. Also from this plate six bonding/grounding wires go to the six cable trays and four wires go to four panels and are terminated on an IG ground bar. In the fifth panel they have an IG bar with the ground wire going to building steel. On the cable tray they used MC cable to wire IG receptacles when they found out that they could not use MC cable for IG receptacles the original contractor came back and installed a ground lug and a wire in the panel and landed it on a lug on the cable tray. He told them that now all the boxes would be grounded.
I told them that the IG wire should be run from XO in the transformer to the MDP and then installed with the feeders to the panels. I also told them the MC could not be used for IG receptacles and that they should replace the wire.
I would like to know if the bonding/ground wires on the copper plate to the cable trays is OK? I consider the wires to be bonding wires.Thanks for any input you guy's have.

 

[bennie]

Re: Grounding TV station

First of all run one ground conductor to the first cable tray, daisy chain with a bond wire to the others. The six separate conductors to the trays can become many ground loops.

Dereck, and others, are skilled in this topic and may add more.

 

[iwire]

Re: Grounding TV station

You have got a mess on your hands, along with what Bennie said I would add you must get the IG circuits sorted out.

You already seem to know what needs to be done.

The IGs must get back to XO with out connections to anything but IG outlet terminals.

And they must run with the circuit conductors, you can get IG MC (a green and a green with yellow). Could you fish new mc cables to the outlets?

 

[bennie]

Re: Grounding TV station

fc: Is this a video and sound stage? Is the transmitter remote from this location?

Please elaborate on the problem with grounds? What exactly is occurring?

 

[gwz2]

Re: Grounding TV station

The IG should be installed with the circuit ( feeders ) and not taking an individual route to the MDP.

I would think that the MDP would/could be the originating point for the IG's.

MC cable with-out an insulated grounding conductor plus the regular grounding conductor is not for IG circuits.

Read 250.146(D) very carefully.

Since you have EMT, assuming from the MDP to each panelboard, that should be OK for the EGC and the IG for each panelboard should also be in the same EMT.

An open run of Bonding Jumber (BJ) would not be desireable from the Wall Copper Bus to each panelboard.

As Always, the EGC and the IG is required to be installed " WITH THE CIRCUIT CONDUCTOR(S) ".
That is, with EACH branch circuit and/or EACH feeder circuit.
See 250.146(D) and 250.134(B).

 

[dereckbc]

Re: Grounding TV station

FC as you already know the IG is hosed. As iwire pointed out the IG originates from the Xo of the transformer and is kept completely isolated downstream from that point. It can be split to separate feeders to serve multiple sup-panels, but it has to be kept isolated and only terminate on the IG pins.

What you describe if I understand correctly will cause noise problems by originating the IG from a different point other than the Xo. In fact it sounds like code violation.

The cable racks are fine left in a radial fashion as long as the ground bar they are terminated too is bonded to the GEC and not depending on earth for a fault path. Yes they are bonding jumpers and not EGC's. So if you are worried about them not being in a raceway, forget about it, its OK.

[ June 10, 2003, 08:52 PM: Message edited by: dereckbc ]

 

[gwz2]

Re: Grounding TV station

This is a SDS system and the Xo or the System Disconnecting Means is NORMANLY the only choice for the GEC and that point should be where the IG originates.

It seems to me that even with the EMT as the EGC to the panelboard(s), for a broadcasting station, that it would be prudent to have a wire ( insulated ) EGC with-in the EMT as the required EGC plus the IG.

 

[fc]

Re: Grounding TV station

The main problem they are having is noise. I say the main problem is all the code violations.This is a TV broadcasting station "Univision". This space was done 3 years ago.The office building they are in I do the work for the owner but did not do this job.The owner did not build the space they let Univision do it themself.Last year they called me in to check out a problem and I seen the way it was wired and toll them they had a mess on there hands. They call the original contractor back the electrical inspector and engineers . All the IG outlets our on the racks so they came up with just installing a ground wire from one of the panels to the cable tray and said now all the outlet boxes are grounded. I told them yesterday that they are wrong.I think there best bet is to not have IG bar in the panels and to land the IG bar right on the panel and to install the EGC to the box of each IG outlet.The best thing I like is that they had a electrical testing lab come in and look at the grounds.This is what they came up with.
"Testing revealed that many 120v circuits are multiply grounded and as a result there are circulating ground currents in almost all pieces of equipment raceways and ground ( green) conductors. With multiply grounded circuits conductors, it only requires leakage,or an inadvertent neutral to ground connection in one piece of equipment - to provide circulating ground current in all conductors."
"All racks grounds showed current flow. At the ground buss in panel DP-Ua the combined cable tray grounds were carryind 17.2 amps! There was 88% to 98% third harmonic content in these ground currents indicating that it probably comes from equipment switching power supplies."
"In summary once there are multiple grounds not just common at the source and there is ANY source of ground current this current will circulate in the ground system which now includes every ground conductor piece of grounded equipment and building steel. Cleaning up this situation os likely a time consuming labor intensive pass time."

That's what the lab report said. Nothing about any violations.
Thanks guy's I think I am on the right track. I hope they will take care of the violations.

 

[dereckbc]

Re: Grounding TV station

FC, if you have 17 amps flowing through EGC's and bonding wires you have some improper wiring methods such as N-G bonds downstream from the Xo point. Switching power supplies cannot cause these high currents. Only improper load wiring will cause it to be this high.

As a data center power engineer I do not recommend any IG circuits unless demanded by the customer. The only way for an IG circuit to be effective is to install it dedicated, only a single circuit, that powers only a single piece of equipment that does not interconnect with any other equipment via a ground, shield, or grounded conductor.

What we sometimes do if required by the manufacture is install a SPG isolated ground plane. We do this by establishing a ground window and isolating all raceways and equipment downstream from the N-G bond or battery return bond to ground. This prevent any outside current from flowing in the isolated ground plane such as lightning, external faults, common mode current, etc.. But the only equipment we do this with is digital telephone switching and radio transmitting equipment. But never for IT equipment because it would be impossible to prevent incidental contact with ground such as conduit, building steel, data/signal cables, etc.

For data centers we still establish a SPG for the SDS systems, but no IG outlets are provided. Instead we supply conventional outlets that are dedicated usually from a UPS/PDU combination. We then construct a raised floor grid under the raised floor if provided, otherwise overhead, and then bond the equipment frames and cable racks. This forms a multi-point ground, which exhibits very low impedance to high frequencies back to the power source and earth. But there is no measurable current flowing in any ground conductors unless someone improperly wires a load, and we have a monitor that sets off an alarm immediately.

I guess what I am trying to say is you have an improperly designed system, improperly installed, or both.

 

[fc]

Re: Grounding TV station

Thanks Dereck
I think they have both a bad design and without a question improperly installed. What concerned me is that they had this electrical testing company come in and as far as I can see does not come up with what should be done. I am going to tell them they should hire a good engineer like yourself to design the right system for there complex.
The first thing that I told them was to eliminate all the improper IG outlets. I would install ground bars in all the panels and land the MC cable grounds to it and land the ground to the outlet boxes and then to the outlets or the can replace 200 IG outlets with new wires. We will see if they decide do anything.
My concern is for people and the owner of the building protection. If they installed the system wrong and is a hazard to the people who work there I am sure he would want to know. Again my thanks to all.

 

[dereckbc]

Re: Grounding TV station

What I find disturbing is how you could get a bad design, improper installation, and a poor audit. Was it all done by the same company? What are the chances of getting three incompetent companies in a row? Was the design stamped by a PE?

 

[bennie]

Re: Grounding TV station

Dereck: As you well know the NEC grounding procedures are not always compatible with sensitive equipment.

The NEC does not recognize current, that creates noise in sensitive equipment, as objectionable.

A grounding design can be done to limit or prevent current flow where it should not be.

Any ground conductor that is earthed in two locations will have current flowing. The current can be from normal load or abnormal outside interference.

[ June 11, 2003, 08:37 PM: Message edited by: bennie ]

 

[fc]

Re: Grounding TV station

Yes it was stamped by the PE. And you can add one more to the list the town electrical inspector who passed the job.I wonder sometimes. During construction I was told that the engineers stayed on the job.
The GC was a large construction company Turner construction.
The biggest laugh is that they would not let me bid the job cause they said our company had no experience doing TV station. Now they want to use us.

 

[iwire]

Re: Grounding TV station

Originally posted by fc:
The biggest laugh is that they would not let me bid the job cause they said our company had no experience doing TV station. Now they want to use us.

That is some satisfaction, I hope you price it accordingly.

 

[fc]

Re: Grounding TV station

Bob

I may get some TV time on there station if the place does not burn down.
Yes I know some of the Turner Tot's well.But I can't blame them for the electrical contractors bad workmanship.I am sure this problem will turn into a nice job for our company.