Isolated Ground questions

[bwyllie]

I know this topic has been discussed before, but I am trying to get a good understanding of this IG system and have a couple of questions:

1) What is the main reason for installing an IG system? and is it really worth it?

2) Is it better to install a hot, neutral, egc and Ig wire to each outlet or would installing Hospital Grade AC cable be sufficient(using the cable sheath as an EGC? Why?


Thanks for your help I appreciate all responses.

 

[Guest]

Re: Isolated Ground questions

It depends on the installation. From the research I have done here and elsewhere I no longer have faith in the IG concept. It may even be a hazard in certain situations (think touch potential). The cost to benefit ratio may be excessive.

If you are running multiple pieces of equipment there are too many ways to compromise the integrity of the IG. It's easy to create unintended & multiple fault paths that get around the IG.

I am not an EE, but my best effort would be to put protection in the meter base. Provide some more protection in the load center. Then have some point-of-use automatic voltage regulation (line conditioning). This scenario is assuming computer or electronic equipment.

With a scope I have witnessed "dirty" power with the ability of to make computer & electronic equipment act in a Poltergeist fashion. I personally do not have this scope testing capability-- but I wish I did.

Clean power is a science, an art, a bit of luck, and fully dependent on your enviornment & application.

There are gaps in my answer. Other answers will have gaps too. Put them all together: Hopefully no more gaps.

 

[dereckbc]

Re: Isolated Ground questions

What is the main reason for installing an IG system? and is it really worth it?

The only purpose of an IGR is to possible prevent common mode noise from entering the circuit PERIOD. However if the origination point of the IGR already has common mode noise, the circuit is useless and will only make the problem worse. Garbage in, garbage out. If common mode noise is a real problem you are better served to use an isolation transformer. Isolation transformers will attenuate or CMR from 40 to 120dB. And IGR CMR is 0dB.

I can tell you the three largest telephone companies that construct and lease data centers for Internet hubs do not use them for their data equipment. Instead they use a PDU (a special isolation transformer supplied via UPS) with a dedicated circuit consisting of L, N, EGC in conduit. They will install them for customers who insist, but no gaurentees come with it, as SG does.

The only practical use I know of is for A/V applications, but some strict rules need to be observed, or it is compromised.

[ October 20, 2003, 02:18 PM: Message edited by: dereckbc ]

 

[iwire]

Re: Isolated Ground questions

Dereck as usual thanks for straight forward answers.

I too have some questions.

We install PDUs like you describe (Typically Lieberts) and some have one ground bar and some have two.

One is bolted to the PDU enclosure and One is "floating" and tied into the unit with a green with yellow stripe conductor.

Forgetting that down stream they end up connected again by what ever means, is this IG bar any different then the other EGC bar.

Or is it just a "placebo" for the customer that thinks it is a necessary or a benefit?

 

[ron]

Re: Isolated Ground questions

Bob,
When specified, manufacturers will stand on their head if told to do so in the specifications to get the project.
Is it possible that this second isolated bar is for bonding the neutral to ground? Is there a neutral connection elsewhere? Other than that, check the install manual, could be many reasons left up to the imagination..

 

[Guest]

Re: Isolated Ground questions

Since Dereck hasn't jumped in I'm going to offer that bar is for the pass-through IG.

Are there three bars?
1. Isolated neutral bar.
2. Isolated ground bar.
3. Ground bar.

 

[dereckbc]

Re: Isolated Ground questions

Bob, not sure I completely understand your scenario. As Ron stated the manufacture will configure the unit anyway the customer wants to make the sell. Here is how I do them; I will try to be brief.

1. Install a GEC to the Xo of the PDU/transformer per 250.30.
2. Install MBJ from Xo to PDU ground terminal. Sometimes this is a buss or a stud.
3. Install bonding jumper from ground terminal to EGC buss.
4. Install bonding jumper from EGC buss to IG isolated bus.
5. Originate EGC?s and IG?s from the respected busses.

There are a couple of variations from one manufacture to another such as there may be only one ground buss that supplies both EGC?s and IG?s, or the EGC bus and IG bus bonded individually to the frame ground, but the concept is the same.

The one point I would like to make is if you are going to have IGR?s there needs to be an isolation/step-down transformer involved, and the IG should originate at the Xo N-G bond point. It is the transformer that provides the CMR and not the IGR. Otherwise the whole point is lost if you separate the EGC/IG downstream from that point as the NEC allows. The kicker is if you use a transformer and run dedicated circuits, the IGR is a waist of money IMO.

Hope that answers your question.

Dereck

 

[iwire]

Re: Isolated Ground questions

What I was thinking in an untrained way, was that the power coming off a PDU fed from a UPS is as good as it gets and are the separate ground bars really a benefit even if we assume they will not be intermixed at the user end of the branch circuit.

I will readily admit I have no formal training in any of this and my opinion is based on what I have seen and done.

It seems the highest end Data rooms I go in do not use IG, but the small to the middle size operations always seem to have someone convinced that their equipment is far to high tech to work with out an IG.

I am presently wiring a convince store, they have me running IGs to security monitors, lottery terminals, ATM, copy machine, FACP, etc. They paid for it so they will get it, but what is the possible benefit?

 

[bwyllie]

Re: Isolated Ground questions

when installing IG circuits is mostly Hospital AC cable being used or a 4-wire IG system?

 

[iwire]

Re: Isolated Ground questions

I am sorry if I got this thread off track on you.

If type AC cable is acceptable in the building then yes we sometimes use Hospital Grade AC cable for IG circuits in the way you describe.

Most of the time we use 12/2 or 10/2 MC IG cable.

It has Black, White, Green and Green with yellow stripe.

To keep things clear we use plain green as the EGC and the green with yellow stripe for the IG conductor.

In this area it is a stock item at the big supply houses.

We have also used 12/3 MC or AC and remarked the red, to green with yellow but that would have to comply with 250.119(B)

Part of 250.119(B)

Where the conditions of maintenance and supervision ensure that only qualified persons service the installation, one or more insulated conductors in a multiconductor cable, at the time of installation, shall be permitted to be permanently identified as equipment grounding conductors

 

[bwyllie]

Re: Isolated Ground questions

what are the pros and cons of using the 4-wire(H,N,EGC & IG)vs using hospital grade AC and utilizing the sheath as the EGC.

 

[iwire]

Re: Isolated Ground questions

IMO it looks better to use the type IG cable as it is clearly, permanently identified at every location.

Other then that I can see no difference, all the methods we have talked about utilize a full size copper conductor for the IG and a code compliant method of providing an EGC.

But many jobs have specs that go beyond NEC minimums, one of those specs is often the mandatory use of a copper grounding conductor.

This rules out using AC as we have talked about and of course type AC is not permitted in a "place of assembly" without a grounding conductor 518.4(A), so IMO that prohibits the use of the green in hospital AC for the IG in these places of assembly or for that matter in an area that already requires the use of hospital grade AC as in a hospital.

[ October 21, 2003, 05:00 PM: Message edited by: iwire ]

 

[bwyllie]

Re: Isolated Ground questions

Thanks for your responses the information is very useful. As an engineer I see no extreme benefit to utilizing the sheath of AC for IG circuits or the conduit as an EGC in a system. If you disagree I would like to hear the reasons.

 

[iwire]

Re: Isolated Ground questions

I am not sure what you are asking.

Are you asking if there is a benefit to using the raceway for grounding?

From your side I would say no, from my side there is a real benefit. Less conductors to buy, install and terminate.

If the contract does not ask for copper ground conductor I will not provide one for free.

If I am doing T&M work I always provide a grounding conductor or if the specs require it I will be sure to provide it. I do not "cheat" on the specs. I follow them to my advantage or theirs.

 

[bwyllie]

Re: Isolated Ground questions

I agree with you on that one. Typically on circuits and feeders we call out for the egc to be a copper conductor, but as cost items or value engineering item I do not object to using the raceway. I was asking if there is any "engineering" reasons why the conductor is better than the raceway.

 

[iwire]

Re: Isolated Ground questions

When you use the sheath or raceway as an EGC you have many more grounding connections.

Obviously true for raceways, but even with Type AC, in a box with four AC cables you have 4 separate grounding connections, the same box with type MC cable all four (five including box bond) grounds make one connection.

IMO that makes the use of the sheath or raceway more prone to a connection failure.

JMO, Bob

[ October 21, 2003, 07:52 PM: Message edited by: iwire ]

 

[dereckbc]

Re: Isolated Ground questions

Originally posted by bwyllie:
I was asking if there is any "engineering" reasons why the conductor is better than the raceway.

Yes, you have a planned path for faults rather than relying on conduit.. Conduit EGC is marginal at best, you are relying on multiple mechancal connections to provide a planned fault path rather than a copper path. Additionaly the conduit provides a EMF/RFI shield, and lowers the planned fault path impedance (in additon to planned copper EGC path). IMO provide a EGC along with a EMT or other 250.118 EGC.

[ October 21, 2003, 10:56 PM: Message edited by: dereckbc ]

 

[dkerwin]

Re: Isolated Ground questions

Whatever 'isolated' design you use, there are ways of monitoring for 'ground faults' - i.e. losses in insulation level beteen the isolated conductor and ground.

Insulation Monitoring Devices (IMDs) and Line Isolation Monitors (LIMs) have been widely used in medical facilities for years. They enable the system to operate with all of the benefits of being 'ungrounded', while providing ground fault monitoring.