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Anonymous Poster #1

How to Test Cabinet Ground

07/23/2013 7:41 AM

We are currently working at a site that has an SRGS already installed throughout the building. We recently add an exothermically weld using a #6 wire to the SRGS. This #6 wire is then attached to the ground bar within the cabinet using a 2-hole lug. The cabinet is then connected to the same ground bar using a #6 wire with one hole lugs. The cabinet door is connected to the cabinet frame using #12 wire; as most cabinet manufacturers do.

We are required to have a total resistance of less than 1 milliohm. The question is, "how do i properly test each connection so that they meet the requirement?"
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PWSlack
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#1

Re: How to Test Cabinet ground

07/23/2013 8:25 AM

Run two wires instead of one. Open one of the wires and measure milliohms at the gap. Connect-up the open wire again, and divide the reading by four.

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PeterT
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#2

Re: How to Test Cabinet Ground

07/23/2013 9:56 AM

Very low resistances are typically measured with a DLRO (Digital Low Resistance Ohmmeter) instrument, often called a "ductor." Its dual tip probes inject a low current across the connection or conductor whose resistance is being checked, and measures the voltage drop caused by the current flowing through the resistance. A simple application of Ohm's Law (done by the DLRO) gives a display of the resistance, often in milli- or micro-ohms.

You can use a DLRO to measure the resistance between the ground bus in the building and the ground bar in the cabinet. First test with only one of the ground cables connected, and then do a final test with both in place. If you don't have a DLRO, they are available for rent from electrical test equipment rental companies, or you can buy one if you need to use it often. Major brands include Megger and AEMC.

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electronick
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#3

Re: How to Test Cabinet Ground

07/23/2013 12:00 PM

try once at the time... by the way, what is an SRGS, and an # conductor? have they square cross section?

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cuba_pete
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#11
In reply to #3

Re: How to Test Cabinet Ground

07/30/2013 2:18 PM

Signal Reference Grid...

This is a "ground" used in addition to the normal safety ground. It may or may not be bonded, depending on the sanity of, and the regulations faced by the locals or AHJ.
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ignator
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#4

Re: How to Test Cabinet Ground

07/23/2013 5:01 PM

Have you checked the wire gauge size for resistance per foot?

#6AWG copper wire is 0.000465 ohms/ft. So 2.15 foot is 1 milliohm.

#12AWG is 0.00187 ohms/ft.

How long are your wires?

Who is specifying a 1 milliohm budget?

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PWSlack
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#5
In reply to #4

Re: How to Test Cabinet Ground

07/23/2013 5:46 PM

<...Who is specifying a 1 milliohm budget?...>

Quite. How abstruse.

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tchendricks
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#7
In reply to #5

Re: How to Test Cabinet Ground

07/24/2013 1:42 AM

At this point, we are required to meet the 1 milliohm standard at the bus bar, at the cabinet frame, and with the 12 gauge that goes from the frame to the door.

Believe it or not, we actually have quite a few that meet this standard. The problem is the remediation to get the other cabinets to meet it.
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PWSlack
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#8
In reply to #7

Re: How to Test Cabinet Ground

07/24/2013 3:58 AM

A curiousity in itself.

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tchendricks
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#6
In reply to #4

Re: How to Test Cabinet Ground

07/24/2013 1:39 AM

Yes, we have checked the wire gauge size.

As far as who is specifying the budget, we are still trying to argue that the MIL standards say 1 ohm but the QA insists it is 1 milliohm.

More to come...
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PWSlack
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#9
In reply to #6

Re: How to Test Cabinet Ground

07/24/2013 3:59 AM

It's a good recipie for installing ground/earth conductors that are substantially oversized.

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cuba_pete
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#10

Re: How to Test Cabinet Ground

07/30/2013 2:15 PM

One milliohm from where to where?

The drop across a solid copper bus bar beyond a few inches will be greater than one milliohm.
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JWthetech
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#12

Re: How to Test Cabinet Ground

07/30/2013 3:26 PM

After reading this several times and reviewing the posts, I think the resistance they are requiring is the resistance of the cabinet to the "2 hole lug" and the cabinet door resistance reading. As others have pointed out, the resistance of the #6 and 12 wire is greater than 1 milliohm. This seems to be a tempest requirement. You may also need to install RF gaskets around the door frame and shield any ventilation openings.
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daveross100
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#13

Re: How to Test Cabinet Ground

01/22/2014 11:46 PM

run 10A and measure 10mv
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cuba_pete (2); daveross100 (1); electronick (1); ignator (1); JWthetech (1); PeterT (1); PWSlack (4); tchendricks (2)

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