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Author Topic: Too isolated ground  (Read 17780 times)

Frank Koenig

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Re: Too isolated ground
« Reply #10 on: November 27, 2013, 11:19:24 am »

the iso ground is not suppose to be bonded to the neutral buss.

250.146 (D) does not agree.  A ground fault current still has to find its way back to the grounded circuit conductor at the service entrance. "Isolated" just gives you license to run the equipment grounding conductor through as many panel boards as you want (without connection) to get to the place where you connect it to the rest of the grounding system. -F
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John Roberts {JR}

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Re: Too isolated ground
« Reply #11 on: November 27, 2013, 12:07:49 pm »

Amen... the safety ground fault current must get back to the breaker panel with low enough impedance that the breakers will open under fault conditions. 

JR
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Matthew Knischewsky

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Re: Too isolated ground
« Reply #12 on: November 27, 2013, 12:49:26 pm »

IMO the biggest advantage for an isolated ground in MOST audio applications is the fact that there is copper wire going from the ground rod right to the receptacle. Can't comment on other jurisdictions but here it's acceptable to use EMT conduit as the ground. Over long runs and many conduit junctions there's more resistance than wire which can cause grounding problems with our gear, especially if the conduit starts to corrode in damp environments. Similar results can be obtained by pulling a dedicated ground conductor through each conduit without doing a true isolated ground system.

And for clarity... no matter if it's an isolated ground system, ground and neutral always have to be bonded together at only one location. For most buildings that one place is the service entrance but if there's additional transformers that one place is at the XO (neutral) of the transformer.
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Mac Kerr

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Re: Too isolated ground
« Reply #13 on: November 27, 2013, 01:12:41 pm »

IMO the biggest advantage for an isolated ground in MOST audio applications is the fact that there is copper wire going from the ground rod right to the receptacle.

How is this a big advantage for most audio applications?

Mac
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Jeff Bankston

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Re: Too isolated ground
« Reply #14 on: November 27, 2013, 02:29:58 pm »

250.146 (D) does not agree.  A ground fault current still has to find its way back to the grounded circuit conductor at the service entrance. "Isolated" just gives you license to run the equipment grounding conductor through as many panel boards as you want (without connection) to get to the place where you connect it to the rest of the grounding system. -F
in los angeles the city electrical inspectors will not allow the iso ground to be connected to the neutral buss in a panel board. the iso ground can be connected to a transformer xo. there has ben much discussion of this over the years among electricians and electrical engineers. i am a commercial journeyman electrician.
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John Roberts {JR}

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Re: Too isolated ground
« Reply #15 on: November 27, 2013, 02:48:14 pm »

in los angeles the city electrical inspectors will not allow the iso ground to be connected to the neutral buss in a panel board. the iso ground can be connected to a transformer xo. there has ben much discussion of this over the years among electricians and electrical engineers. i am a commercial journeyman electrician.

I am unclear about terminology. What is a transformer "xo"? Is that possibly the center tap on the pole distribution transformer delivering the two polarities of 120V that measure 240V across the two legs? If yes, isn't that center tap connected to neutral in the power panel?

I assume buss is same as electrical bus, not the brand of fuses.

If the isolated ground is not connected to neutral it may be ineffective as a safety ground to trip breakers. Are RCD/GFCI required for such iso-ground installations? If not they should be.

JR

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Jeff Bankston

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Re: Too isolated ground
« Reply #16 on: November 27, 2013, 03:33:10 pm »

I am unclear about terminology. What is a transformer "xo"? Is that possibly the center tap on the pole distribution transformer delivering the two polarities of 120V that measure 240V across the two legs? If yes, isn't that center tap connected to neutral in the power panel?

I assume buss is same as electrical bus, not the brand of fuses.

If the isolated ground is not connected to neutral it may be ineffective as a safety ground to trip breakers. Are RCD/GFCI required for such iso-ground installations? If not they should be.

JR
there is way too much to type regarding the XO so i will post a simple reply. the xo is where you get your neutral from a transformer. the xo must be grounded. the xo in the foto has a white circle around it. i'm not trying to be a smart alec but if you really want to learn about the technical aspects of all this electrical engineering school is a must.

i have never had gfic recepticals specified for use on an iso circut. there are special recepticals that you must use. the ground hole will have a green dot next to it. the metal frame of the receptical is not connected to the ground hole.
« Last Edit: November 27, 2013, 03:43:18 pm by Jeff Harrell »
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TJ (Tom) Cornish

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Re: Too isolated ground
« Reply #17 on: November 27, 2013, 05:08:26 pm »

there is way too much to type regarding the XO so i will post a simple reply. the xo is where you get your neutral from a transformer. the xo must be grounded. the xo in the foto has a white circle around it. i'm not trying to be a smart alec but if you really want to learn about the technical aspects of all this electrical engineering school is a must.

i have never had gfic recepticals specified for use on an iso circut. there are special recepticals that you must use. the ground hole will have a green dot next to it. the metal frame of the receptical is not connected to the ground hole.
With respect, I'm pretty sure JR understands isolated grounds and bonding.
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Frank DeWitt

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Re: Too isolated ground
« Reply #18 on: November 27, 2013, 05:18:44 pm »


If the isolated ground is not connected to neutral it may be ineffective as a safety ground to trip breakers. Are RCD/GFCI required for such iso-ground installations? If not they should be.

JR

A isolated ground outlet is grounded.  The ground opening in the outlet is connected to ground (isolated) and the box and cover are connected to ground (conduit / building steel)  From a safety standpoint there is no difference.  Only the path to ground is different. 

BTW it may be worth noting that GFI outlets do not need a ground to operate.  They compare the current on the hot and neutral and if they are sufficiently different, the device trips.
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Kevin Hoober

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Re: Too isolated ground
« Reply #19 on: November 27, 2013, 05:29:42 pm »

the xo is where you get your neutral from a transformer. the xo must be grounded.

Looks like everyone is catching up one the board today!

One thing the install got right: there is actually green tape with the yellow on both ends of the wire going to the rod.

I was referring to XO when inappropriately using the term "neutral bus". 

Would it be common to lose the incoming ground in favor of local building steel?  This is a remodel in a warehouse (not much building steel)

My understanding is that code requires any ground (isolated or not) to be bonded to the neutral (XO). Unbonded is certainly unsafe, as others have stated, as there would not be a fault current return path.  I do not see how 3 of the 4 examples, in your first post, meet this criteria--am I missing something?

Not a journeyman electrician,
Kevin H.

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« Last Edit: November 27, 2013, 05:46:00 pm by Kevin Hoober »
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Re: Too isolated ground
« Reply #19 on: November 27, 2013, 05:29:42 pm »


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