Ground Rods all over the place.

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mkgrady

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I wired a couple of boilers today in a small manufacturing facility. They are insisting that a ground rod be installed through the slab at each boiler control panel. I asked them why they wanted the rods and they indicated they had problems in the past where fault currents burned up some of their equipment.

The building has a 120/208 system. All circuits are in emt or GRC (the older stuff). My contact explained to me that they don't have the green wire in their circuits so they install rods at all equipment. I explained that the conduit is the grounding conductor. I couldn't convince him that all their ground rods were unnecessary. He said two electricians told him it's a good idea.

They are driving the rods and are running #6 CU, thhn from the rods to the equipment. They asked me where they should connect the #6 at the boiler control panel. The BCP only requires a 15 amp 120 volt circuit so it's not like the #6 will fit in the tiny little connection box. I told them if they insist on connecting the BCP to a rod they can just drill a hole in the cabinet and install a lug on the outside.

My questions are: What good is the rod if the emt feeding the equipment is in tact? Are there any detriments to adding a ground rod at equipment? Does it violate the code in any way?

Thanks, Mike
 
What good is the rod if the emt feeding the equipment is in tact?
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Ground rods can not be used in lieu of an equipment grounding conductor.(See 250.4(A)(5))

A grounding electrode does not do any good in clearing ground faults for equipment. The purpose of a grounding electrode is to limit the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines. (See 250.4(A)(1))

Are there any detriments to adding a ground rod at equipment?
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No.

Does it violate the code in any way?
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Provided that the equipment has a properly installed EGC, you can install an auxillary grounding electrode in accordance with 250.54.

Chris
 
mkgrady said:
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My questions are: What good is the rod if the emt feeding the equipment is in tact?
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Gives the customer a warm fuzzy feeling :)
mkgrady said:
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Are there any detriments to adding a ground rod at equipment? Does it violate the code in any way?

Thanks, Mike
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As raider1 states, no problem.
 
I would think if they don't have a good grounding conductor and they add a ground rod a ground fault would not draw enough current for the CB to open.
 
mkgrady said:
I would think if they don't have a good grounding conductor and they add a ground rod a ground fault would not draw enough current for the CB to open.
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You are exactly right.

For example, if a ground rod had a resistance of 25 OHMs and we had a 120 volt source go to ground on that rod, the current would be 4.8 amps. That is obviously not enough current to trip a 15 or 20 amp overcurrent device.

Chris
 
mkgrady said:
So, is the only benefit that surges may be limited thereby protecting electronics in the equipment being wired? Would they also get some protection from lightning?
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I don't really see the additional rod providing any real protection from either of those problems.
 
FirstChoiceElectric said:
I may be misunderstanding the code but if that where done wouldn't it then be have to all be interconnected and brought back to the gec system. I realize its not right but if your going to do something wrong atleast do it wrong right :grin:
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Not exactly.
250.54 Auxiliary Grounding Electrodes.
One or more grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or 250.53(C) or the resistance requirements of 250.56, but the earth shall not be used as an effective ground-fault current path as specified in 250.4(A)(5) and 250.4(B)(4).
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Normally this can't hurt anything, but it gives the owner/tenant a false sense of safety, and I can think of one scenario where it might actually backfire. suppose the ground is extremely non conductive, but the concrete at the surface is moist and conductive. suppose there is a fault to the ground rods, but no egc and hence no return fault to source, no tripped breaker, but essentially a live surface where someone might get hit.

likely ? no. possible ? yes.

convice them to install proper bonding.
 
If the grounding electrodes are not bonded together each additional electrode will provide a path through-the-equipment for transient overvoltage events. A properly installed EGC provides a safe path. Without the EGC the surge will find it's own path.
 
Pierre C Belarge said:
This is really good advice...make sure that you are not liable for any buried items...call for a markout or have the company liable for such circumstances.
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Very true. Have them do the drilling and driving and you make the connections. Or convince them to let you just pull an EGC back to the panel. And charge accordingly
 
A ground rod at a machinery location such as yours will actually make the equipment more liable to be damaged by lightning. The ground rod will create a difference of potential in a lightning strike and can induce current into the machine. There is a study by the EPRI that supports this and Mike Holt has a good graphic to illustrate it.

I call these Time and Material ground rods.
 
I think it's important to understand the differences between, grounding, bonding and earthing. The process of driving an isolated ground rod and using it as the only means of equipment grounding is prohibited by the 08 NEC (250.54).

There's an excellent article on this subject in Jan 2009 issue of EC&M.

The article basically says that in an attempt to find ways to reduce the effects of stray currents, some equipment installation designers, use earthing as a method to "isolate" their equipment.

Obviously the danger here is you remove the "low impedance path" back to the power source, which is required to trip the OPD.

I think another interesting point, and it is allowed by the NEC, is using metallic conduit as the "low Impedance path" back to the power source.

I would argue, that depending on the quality of the installation, metallic conduit may NOT be a "low impedance path"

It's a good article worth searching for. Authors Larry Ray and S. Frank Waterer.
 
FirstChoiceElectric said:
I may be misunderstanding the code but if that where done wouldn't it then be have to all be interconnected and brought back to the gec system. I realize its not right but if your going to do something wrong atleast do it wrong right :grin:
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There is no requirement for this rod to be made part of the GES ,.. it is required to be connected to an equipment grounding conductor as found in 250.118 ,.I'm not sure a lug on the boiler would comply...
 
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