Raised Floor Grounding - Non-Stringer

[hbendillo]

I've been doing a lot of research about when and how extensive the grounding of a raised floor needs to be. Related to that is the necessity of a Signal Reference Grid under a raised access floor. I think the SRG is largely unnecessary these days but there are still some applications where I use them or am directed to use them.

Anyway, my question is about raised flooring systems that don't used bolted stringers just isolated pedestals on which the floor is installed. Most of these flooring systems use floor tiles that metal on the bottom side and in affect when all floor tiles are installed are bonded together. But even if there weren't, I am not sure that is a necessity. But I am not getting a clear picture on the issue.

Well I am involved in a job where the raised floor system was changed from a bolted stringer type to a non-stringer type. The electrical contractor has issued a change order to install a ground system whereby each pedestal is grounded. Is that really necessary here?

Application: Office building with a technical support center that basically has open office areas and rooms with computer work stations. They also have flat panel video displays throughout. The main servers for the data delivery are installed in another area of the building.

I'd like to hear some opinions on whether we need to ground every pedestal or general comments about raised floor grounding for this application.

Thanks.

 

[hbendillo]

I can see this is a tough question to answer. I did a search on the forums for "raised floor" and found some commentary but no definitive answers. It seems the question to answer is whether the floor system is likely to become eneregized. After that, where and how many times should it be bonded. I am leaning to the "not likely to become energized" conclusion with a I'm still going to bond the floor in a couple or four places for additional safety anyway. The grounding system proposed by the contractor, bond every floor pedestal, is unnecessary.

 

[infinity]

Do you have a link to this raised floor product?

 

[masterinbama]

Do you have a link to this raised floor product?

Here is an image of 3 types of raised floor.

 

[__dan]

Three code references:

The floor with the bolted stringers could be a SRG. The floor with pedestals only is probably not an SRG. The authority may be a letter from the manufacturer. Neither are listed in 250.118

To be brief, simple bonding of the floor to ground, the PDU common grounding and bonding busbar, one pedestal seems maybe enough to say you tried. The floor surface, the vinyl tile, may be mostly an insulator, so, no exposed metal. In places where other equipment requires equipment or "earth" grounds, IMO, the floor is not a grounding conductor.

250.118: grounding conductor types. raised floor systems is not specifically listed and unless the floor system has a UL listing as an approved grounding conductor, IMO, the floor system is not a grounding conductor for the purpose of meeting grounding conductor code requirements. The floor could be be a supplemental ground conductor, but not a code legal one. You can bond to the floor but not use it as a grounding conductor when required.

645.15 splits the hairs. The floor could be a non code required signal reference grid or structure that requires bonding to the supplied equipment grounding system. The floor is not something that things are grounded to, the floor is something that may itself require attachment to equipment grounding conductors. There's some arguement if 645 applies voluntarily or by requirement. Is it an IT space complying fully or intended to comply with 645 ?

110.3 (B) Look at the literature coming with the rack mounted devices and you will likely see manufacturers requirements to attach "earth grounds" to the threaded location indicated. IMO, an earth ground and an equipment ground are two seperate things. Earth ground connects to the earth by the grounding electrode via the grounding electrode conductor. There is an easy way to do this, ie, run a wire from point A to point B, however, if you are required to comply with the additional strictness of grounding electrode conductor requirements, you will never guess and run the wire correctly. I'm in an action on this one right now, so no disclosure of my easy way. The cycle of trying and failing has not yet completed.

If you are an electrician and want to provide work correctly, the code helps you here. If you want to avoid additional work, the code could expose you to unforseen and unrelated liability, from, say, IT equipment failing to work correctly and the investigation reveals earth grounds per code requirements were not provided as indicated.

 

[__dan]

The EC should write a change order to run an "earth ground" to every rack mounted device that has an earth ground attachment point or an earth ground requirement in the manufacturer's literature.

That is a change order. There is also an easy way to do it correctly. Grounding every pedestal leg does, IMO, nothing. And the floor cannot per code be used a conductor connection to "the earth".

 

[suemarkp]

How do you treat rack equipment that has a screw terminal with the ground symbol, but it doesn't say "earth" (just says "ground")?

If a rack or piece of rack gear actually says "earth ground", is there any reason an EGC cannot be used to get you to earth (the main bonding jumper should provide a path to the ground electrode system)?

Is there any technical reason why some rack equipment would need a true GEC type of connection? I can see how the build up of of static electricity could be an issue in some equipment. But I would think an EGC would have plenty low impedance to dissipate it.

 

[__dan]

How do you treat rack equipment that has a screw terminal with the ground symbol, but it doesn't say "earth" (just says "ground")?

If a rack or piece of rack gear actually says "earth ground", is there any reason an EGC cannot be used to get you to earth (the main bonding jumper should provide a path to the ground electrode system)?

Is there any technical reason why some rack equipment would need a true GEC type of connection? I can see how the build up of of static electricity could be an issue in some equipment. But I would think an EGC would have plenty low impedance to dissipate it.

645.15 has a requirement for rack grounding that is independent of the plug and cord connected equipment. The rack could have no powered devices in it, like a rack for patch panels, and 645.15 requires a rack equipment ground connection. If the device requires only an equipment ground, it is in the cord and plug connection. The EGC connection from the powered and grounded device to the rack ... the rack could be equipment grounded independently per 645.15 or to make the connection from the grounded device to the rack you would have to comply with code requirements for ground connection, clean surfaces, paint removal, paint piercing listed grounding hardware. Do you want to scrape off the powder coating where surfaces bolt together or do you want an independent rack ground?

Rack powered equipment, their power supplies would nominally be seperately derived and require a grounding electrode conductor, but there is an exception written in where the IT rack power supplies are not considered seperately derived for the purpose of applying the code. The point is you may comply with earth grounding requirements without being required to do so.

Devices which specifically and literally say "earth ground" in the literature, I just saw this on an APC rack mounted static switch, but really just every piece of rack hardware has a seperate threaded location, stud or insert, with an earth ground symbol on it. Look at what is intended, two things:

1. All the devices are intended to have a connection to ground where the ground reference level is identically the same, even in distant remote parts of the building. The intended 0 volt reference is at the connection to the "earth" not at a remote panel equipment ground busbar, which is noisy. Item #1. put all the devices on the same reference point for ground, which in a big system, is the earth. Apply the same system consistently, do not have devices referencing different ground systems.

2. If the earth ground is specifically required per 110.3 (B), you may be required to meet the stricter requirements of running a grounding electrode conductor compared to running an EGC, ie, minimum size per T250.66 is #8 cu, no splices or splices using irreversible hydraulic crimps, bond bushings entering and exiting metal boxes. Recall, the power supplies are nominally seperately derived and would fall under the reqirements of 250.30, but an exception is written not requiring this. IMO, the intent is there but not the requirement. The manufacturer may add the requirement back in, in their installation literature.

The common point of EGC and GEC connection is the transformer secondary common grounding and bonding busbar. You may pick up an earth ground connection there if the busbar has the transformer GEC to the earth electrode. Per 250.30 (A) 4, a 3/0 cu run as a GEC can be tapped with hydraulic crimps to provide a GEC to multiple or group devices and equipment.

The system clearly requires grounding in addition to the basic EGC system, whch can run to electrically noisy, seperated, and remote, sub panel EGC busbars. In a large system you can run a test conductor from one remote panel EGC busbar to another remote panel EGC busbar and measure a current flow between them. The code forbids this per 250.6 and provides the remedy with independent earth grounding were all the earth grounded devices are supposed to be referenced to the same 0 volt reference point and system. EGC's and GEC's have two seperate purposes and often run to two different places.

I should charge per word .

There is an easy way to bring GEC earth grounding to the ECG grounded devices. That method is above.