IT (Impedent Neutral) grounding systems

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Hyradix Rick

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My whole concept and understanding of proper grounding is turned upside down when I try to understand the IT grounding system. We are installing our equipment at a client site that has this IT-N type ground system. The power design of this piece of our equipment is based on the TN-S system. I have a 3 phase 380 VAC connection with all our load grounding going to a single point which will connect to the customers neutral ground which is designed to go directly to our equipment? supply transformer?s neutral ground connection. Because this IT system now has high impedance, if a piece of equipment has a short circuit to ground, my protective devices will not trip, it will only trip if the fault is a phase to phase fault, is that correct? They do have an insulation monitoring device that they manage. Does this device detect the "fault" and interrupt the source circuit? I also have a 380-110 VAC transformer as one of our loads that feed heat tracing and heater circuits. Should I then use an earth ground for these secondary circuits since it is a galvanic separation and not go to the clients grounding system? Any thoughts that you could share with me on IT-N systems would be greatly appreciated
 
Hyradix Rick said:
My whole concept and understanding of proper grounding is turned upside down when I try to understand the IT grounding system. We are installing our equipment at a client site that has this IT-N type ground system. The power design of this piece of our equipment is based on the TN-S system. I have a 3 phase 380 VAC connection with all our load grounding going to a single point which will connect to the customers neutral ground which is designed to go directly to our equipment? supply transformer?s neutral ground connection. Because this IT system now has high impedance, if a piece of equipment has a short circuit to ground, my protective devices will not trip, it will only trip if the fault is a phase to phase fault, is that correct? They do have an insulation monitoring device that they manage. Does this device detect the "fault" and interrupt the source circuit? I also have a 380-110 VAC transformer as one of our loads that feed heat tracing and heater circuits. Should I then use an earth ground for these secondary circuits since it is a galvanic separation and not go to the clients grounding system? Any thoughts that you could share with me on IT-N systems would be greatly appreciated
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Sounds like an Isolated Ground system.

You would bond the common, single, insulated grounding point to a single isolated ground electrode or group of electrodes. You make either:

1./ A single connection that is disconnectable to the regular interconnected grounding system. You are ellowed to disconnect this bonding point IF there is objectionable current flow or if it causes the IT equipment to malfunction.

2./ Install a high impedance bond between the two grounding systems to minimize ojectionable current flow.
 
I do not know the terms ?IT-N? or ?TN-S.? But I think you are talking about a ?impedance grounding neutral system.? These are mentioned briefly in 250.20(E), and are covered in detail (for your voltage level) in 250.36.

They are designed to enable continued operation on a ground fault, and can only be used in specific circumstances (one of which is that ?continuity of power is required?). There must be an alarm system to warn the operators or maintenance personnel of a ground fault condition. This is intended to allow time to locate and isolate the fault, or to conduct an orderly shutdown of the equipment served.
 
Have a look at this Wikipedia article on earthing systems that explains the nomenclature, which is used everywhere in the world, except the USA. As noted by others, you may recognise this as a high impedence grounded system, or HRG.

I'm not quite clear from your post; you mention a transformer, is that part of your equipment or the customers supply transformer?
 
OK, I read the Wikipedia article, but what is the N in the IT-N of the OP? IT appears to be an ungrounded or high impedance grounded system. This kind of system has no neutral.
 
IT Grounding Systems

IT Grounding Systems

Thank you all for your input so far.:D

I believe it is just IT, not IT-N. Sorry about the confusion (Hence this posting) I did look at the Wikepedia article and NEC article 250.36 and I do understand what it is, but I still can't fully grasp what the ramifications will be if I hook up to this system when I have designed this unit for a TN-S and TS-C-S. Do I have to worry about ground loops for my instrumentation and controls system (which is part of the 24 VDC system in our unit)?

To answer one of the forum's questions - the transformer is part of our equipment, not the customer. We have a lot of 120 VAC 1 phase loads in addition to the 380 VAC 3-phase loads. Again, since the secondary of this transformer is separated from the main 380 VAC load, how do I treat the neutral from this main 120 VAC system. Do I also connect it to the IT system connection? I also have equipment (pipes, vessels, instruments, panels, junction boxes) that are bonded to ground. I usually made sure they were bonded to a grounding rod next to the unit. Do I make sure that I bond to this new IT ground connection and not use an earth ground?

Thanks again for the input- The fog is starting to clear.
 
Since its your transformer, you can bond one end (or a centre tap) of the secondary of said transformer to the IT ground wire, which will include the metalwork of your machine. The 120V 'chunk' is then a TN-S system, its just like any other SDS.

Your 380V parts will be protected only by the IT system existing protection in the case of a phase to ground fault.

One of the advantages of an IT system is that it can continue to operate in event of a phase to ground short, so the monitoring system often does not cause breakers to trip, but simply a warning signal to be activated. The fault can then be inverstigated (and maybe fixed) without downtime. Of course, if a second fault occurs, it can be really dangerous to any person who gets in its way.

One has to ask; why do they have an IT grounded system? There may be some good reason which means you would want to make your 120V chunk IT also, which is different to the advice I've just given...
 
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