Enclousure & Panel Grounding

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dxjenkins

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I understand the need to ground conductive sub-panels & acess panels that contain electrical equipment. But why is it required to ground similair panels that pianted. I often see painted panels with a small masked area for bonding the panel to the ground bus. The purpose of the ground is to provide a low impedance path to operate the over current or protective device. However, if a non conductice surface, like paint, comes in contact with a live wire, the grounding path would not be completed. What am I not understanding?
Thanx
 
Re: Enclousure & Panel Grounding

I think paint offers a substantial amount of impedance, but I don't personally consider it to be non-conductive, despite what 250.12 says. If you were to take an energized conductor to a bonded, painted surface, I still beleive the OCPD would trip. Maybe I'm wrong though.
 
Re: Enclousure & Panel Grounding

Having done a lot of welding (the intentional kind :D ) I can tell you most paint is an insulator.

If I am welding a well painted (or rusted) surface I will have to strike the rod hard enough to break through the paint or rust, once the arc is established it will continue as the paint burns away.
 
Re: Enclousure & Panel Grounding

Paint is not an approved insulator. Any scratch and whatever insulating value paint has is gone. 120 volts or more will more easily penetrate paint than the 50 volts a welder has for open circuit voltage.
 
Re: Enclousure & Panel Grounding

Derwith,
Paint is not an approved insulator
Click to expand...
a piece of tire isn't either, but it works pretty well as one. :D

I know this isn't the issue, but the word "approved" got my attention. ;)

Roger
 
Re: Enclousure & Panel Grounding

What am I not understanding?
Click to expand...
The intent of the bonding requirements is to establish an equipotential plane to limit the voltage between all non?current-carrying parts of electrical and nonelectrical equipment.
 
Re: Enclousure & Panel Grounding

Roger,
a piece of tire isn't either, but it works pretty well as one.
Click to expand...
Actually it doesn't. Have you ever seen high voltage lines on a vehicle? The tires conduct enough to get hot enough to ignite. There is a lot of carbon black in the rubber used for tires.
Don

[ September 11, 2004, 11:51 PM: Message edited by: don_resqcapt19 ]
 
Re: Enclousure & Panel Grounding

I think that Bob's post hit another subtle idea.

"I will have to strike the rod hard enough"

The same goes for any incidental contact that a live conductor may have during the lifetime of it's useage. The fact that the paint surface can be damaged, even just with age, is the reason for the bonding.
 
Re: Enclousure & Panel Grounding

I guess I was a little off track with my post.

I was trying to point out that paint is an enough of an insulator that it must be removed for a good contact. :)

In no way did I mean it was a acceptable insulator to protect people from live parts. :eek:

[ September 12, 2004, 02:55 PM: Message edited by: iwire ]
 
Re: Enclousure & Panel Grounding

:p
Wow! Glad to see I was not the only one pondering this. Thanx for all the input.

What I am taking away from this discussion is that if a live conductor were to land on a painted surface, it may not necessarily trip the OCPD. If the conductor hit an area where the paint was missing or the voltage was high enough to breakdown any insulating value the paint had, then the OCPD would operate. So the bonding of a painted surface is really to account for the latter case where the paint fails. Of course I not saying that one should rely on paint as protection, I am just making the point that the circuit is not really complete as it is.

Dxjenkins
 
Re: Enclousure & Panel Grounding

The primary reason metal panels and enclosures are grounded, painted or not, is to protect anyone touching those boxes.

If the box was "live" because of a wire, or other inside component, shorted to the box, the overcurrent device would trip and prevent the box from remaining "live" and electrocuting someone.
 
Re: Enclousure & Panel Grounding

Mr. derwith:
I am in 100% agreement with your statements and the theory and reason behind grounding and bonding. You are missing the spirit of this discussion, however. I am merely challenging the idea that the OCPD will be able to operate through a circuit path that contains paint as part of that path.

"The primary reason metal panels and enclosures are grounded, painted or not, is to protect anyone touching those boxes.

If the box was "live" because of a wire, or other inside component, shorted to the box, the overcurrent device would trip and prevent the box from remaining "live" and electrocuting someone."
 
Re: Enclousure & Panel Grounding

If the loose wire or bad component touches the box that is grounded and doesn't trip the overcurrent protective device (OPD), it doesn't matter.

Anyone touching the box will not get electrocuted.

It can be argued that a high impedance short to ground will not trip the OPD and that some voltage may or may not be applied onto the box. This won't last long and will either short completely or clear itself.

Almost all boxes and enclosures over 6x6 are painted. I have accidentally shorted wires to painted metal boxes many times and have almost always got the sparks and tripped the OPD.
 
Re: Enclousure & Panel Grounding

Don, what voltage level are we talking about in this thread?

An MV insulated cable has it's limits, and at some point it's dielectric strength will not contain elevated HV limits, even with shielding.

As far as a tire composition of rubber, carbon, nylon, or what have you containing 600 volts or below, I don't think there would be a problem, of course there are cheap tires out there. :)

Roger
 
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