Grounding of computer room power distribution unit

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dennis323

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Mike,
I have a question concerning the proper procedure for grounding computer room PDU's (Load center, or Power distribution unit). Keeping in mind that the PDU's have a transformer housed within the unit. Voltages are 480volt 3phase
to primary of transformer. Secondary of transfomer is 208/120 volts 3 phase
four wire. I have reviewed NEC section 645-15 and need interpretation as to the meaning of that section.
Thank You.
 

jwelectric

Senior Member
Location
North Carolina
This section will keep referring you around the code book until you end up at 250.30.

I may not be the Mike that this question was directed toward but I gave an answer anyhow.
 

don_resqcapt19

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There may be no need to look at 250.30 for this installation.
645.15 Grounding
All exposed non–current-carrying metal parts of an information technology system shall be grounded in accordance with Article 250 or shall be double insulated. Power systems derived within listed information technology equipment that supply information technology systems through receptacles or cable assemblies supplied as part of this equipment shall not be considered separately derived for the purpose of applying 250.20(D). Where signal reference structures are installed, they shall be bonded to the equipment grounding system provided for the information technology equipment.
Don
 

iwire

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Location
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Don it has been my experience that it is rare for this to happen

through receptacles or cable assemblies supplied as part of this equipment

Most times (but not all times) the wiring from the PDUs to the equipment is field installed EMT or MC or LFMC.
 

dereckbc

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Location
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Don, I rarely disagree with your interpetation of code, but I do on this one. The poster is asking about the Xo of a SDS transformer, not a external equipment frame ground. 250.30 applies IMHO, nor would it be wise to deviate from a technical or safety point of view.

Edited for grammar.
 
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don_resqcapt19

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Dereck,
In general I agree that 250.30 applies, I was just pointing out that in the case of a listed PDU connected to the equipment by receptacles or cable assemblies that are supplied with the equipment, that 645.15 says you don't have to comply with 250.30. It appears that under this rule, there is no requirment to make the secondary of the PDU a grounded system.
Don
 

dennis323

Member
Grounding of PDU's

Grounding of PDU's

I would like to get Mike Holts' input on this matter.
I got several good replies, but no clear answer.
Your help is greatly appreciated.
Thank You.
 

dereckbc

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Location
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Dennis let me ask you this respectively. Why would you even consider not bonding the Xo to the GES? In my 30 or so years of designing data centers and telephone offices, it is not even given consideration except on 130 VDC systems, which is for an entirely different type of system for outside plant operations.

What we usually do with variations is construct what is called a Signal Reference Grid (SRG) with either the raised floor tile stringers, or a grid of 2/0 & 6 AWG conductors under the raised floor with strategic ground bus bars. All the bus bars are bonded to every steel column, and a dedicated 250 MCM cable going directly to the GES. Then each PDU Xo is bonded to one of the bus bars under the raised floor we call Raised Floor Bus .

Then every rack that requires a 120 VAC circuit gets a standard Solid Grounded receptacle, and if the equipment manufactures or customer requires a signal ground it is readily available via the SRG located directly under the floor tile.

I well say this with Ethernet, optical, and other forms of balanced signal transmission, the need for any SRG has all but disappeared completely. New data centers no longer employ SRG?s, but the PDU?s are still referenced to the GES by some means of a permanent planned connection.

IMO the SRG is what the provision of 645.15 is referring too. I have never witnessed a planned SRG that was not bonded to the GES for the facility. At least not one that actually worked.
 

dereckbc

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don_resqcapt19 said:
It appears that under this rule, there is no requirment to make the secondary of the PDU a grounded system.
Don
If this is done then some other means of conductor protection and OCPD would have to apply. Agree?
So something like installing OCPD on both circuit conductors would be needed, plus a monitor sytem to detect a ground. That is pretty expensive with not much benifit IMHO.

I have used PDU's using balanced power or TECHNICAL POWER 120/60 (article 647), but that is still a grounded system, just no grounded circuit conductor to noise things up.

Edited for grammar and spelling errors.
 

don_resqcapt19

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Dereck,
If this is done then some other means of conductor protection and OCPD would have to apply.
Why? It just becomes an ungrounded system just like any other ungrounded system. I'm not sure why you would want to do this, but that appears to be what 645.15 is telling us. Also this could only be used in rooms where Atricle 645 is complied with.
Don
 

dereckbc

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Location
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don_resqcapt19 said:
Dereck, Why?
To protect the circuit conductors. In a delta system you install OCP on all the circuit conductors. Since the Xo would be floating, I assume you would need OCPD on the common what was the neutral conductors. I am in uncharted waters here, as I have never experienced anything like this.

I may be getting off on a wrong tangent, but I have never seen double insulated PDU's, raceways, equipment rack/cabinets, etc. I don't know how you could do it and must be overlooking something. Technically I can see how it would work running the three basic conductors of P, N, & EGC (couldn't call them that), but how would you get them from point A-to-B keeping everything double insulated?

Maybe Ron or Sam can chime in and offer an opinion and shine some light on it.
 

don_resqcapt19

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dereck,
Since the Xo would be floating, ...
Why does it have to have XO? You could use a 3 phase ungrounded 120 volt delta or some other voltage system. It appears to me that the code is giving the manufacturer the option of desiging a listed system that is not grounded as required 1n 250.20(D). Maybe they don't even exist, but the option is there. This rule is just an extension of the fact the NEC does not apply to internal wiring or equipment. The extension is the fact that the code rule lets different pieces of equipment interconnected with cables supplied with the equipment to be considered as one piece of equipment.

I just read the fine print note to this rule. Now I am really confused as the FPN says that the product standards governing this listed equipment ensure that it complies with Article 250.

Don
Don
 

dereckbc

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don_resqcapt19 said:
Why does it have to have XO? You could use a 3 phase ungrounded 120 volt delta or some other voltage system.

Now I am really confused as the FPN says that the product standards governing this listed equipment ensure that it complies with Article 250.

Good point Don. I concede your interpetation is correct. However I have never seen a PDU that didn't have an Xo with 208/120 output. All user equipment I have ever seen like servers, router, hubs, etc either use 208, 120, or -48 VDC. Never have I seen any equipment designed to operate 120 delta or 120 P-P. Those would be some very $pecial custom-made animals.

Glad to hear you are confused :) We agree on emotions.
 

dennis323

Member
dereckbc,
I guess I should give a little more detail. First, and I will appologize if you know this but a PDU has a transformer, Main breaker, sub mains,3 or more 42 circuit panels, and is all self contained in one box. We feed the PDU from a UPS distribution panel 480 volt 3phase and a ground. (Primary side of transformer)
The ground is terminated on a supplied ground bar and to the enclosure. Of course the secondary of the transformer is factory wired and feeds the sub mains which feed the 42 circuit panels. Now the question, We have been told that a new and seperate ground is required in addition to the gorund that was brought in with the feeders to the PDU and it must go to building steel. (the ground with the feeders also goes to building steel). The reasoning is because the PDU is a seperately derived system per 250-30 needs a seperate ground. Does 645-15 apply or is my interpertation wrong.
 

don_resqcapt19

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Location
Illinois
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retired electrician
Dennis,
The only possible way that 645.15 would apply is if all of the cables between the PDU and the equipment were supplied as part of the PDU. Is that the case? Also is this equipment installed in a room that complies with all of the requirements of 645.4? If the answer to either of these questions is no, then you need a grounding electrode connection on the secondary side of the PDU per 250.30.
Don.
 

dereckbc

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Location
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dennis323 said:
dereckbc,
I guess I should give a little more detail. First, and I will apologize if you know this but a PDU has a transformer, Main breaker, sub mains,3 or more 42 circuit panels, and is all self contained in one box. Does 645-15 apply or is my interpretation incorrect.

Been doing it that way for many years as anyone else designing data centers.

Short answer your interpretation is incorrect. The ground being supplied is an EGC (equipment ground conductor) ran with the primary circuit conductors derived from 250.122. The secondary is a Separately Derived System and requires a Ground Electrode Conductor and Bonding Jumper just like any service from the POCO. This falls under 250.30 for SDS.

Since you are not using any odd-ball configuration like Don and I have been bouncing back-n-forth, it is just a Plain Jane 208/120 grounded system like everyone else in the industry uses and 645-15 has no bearing on it.

As to where the GEC goes is really a design issue as I discussed earlier using the SRG. 250.30(4) states shall be as near as practical to and preferable in the same area as the GEC connection to the system The GE shall be the nearest one of the following:

1. Effectively grounded structural metal member of the structure. (Building Steel)
2. Water pipe within 5 feet of building entrance.
3. Other electrode as specified by 250.20.

Edited for grammar errors.
 
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