Neutral

Status
Not open for further replies.
R

rswolff001

Member
I probably already know the answer but I gotta ask anyway........client is building a skid......their customer has an existing 220vac 3-phase receptacle with a ground connection. (4-pin wall receptacle) The skid will require both 220v and 120v.....they've asked if I can use one leg and use the ground as the neutral side. I have said no, that can't (or shouldn't) be done. They indicated they've done before and it should be ok.

And the answer is?

Thanks, Bob
 
You are correct. Doing the bootleg neutral was done in the past, but the danger is if the ground system is not well connected, then you get a floating neutral*, or worse, a HOT chassis!

*and this can lead to the 120 items getting either low voltage or HIGH voltage, neither of which is good for the item.
 
Frank DuVal said:
You are correct. Doing the bootleg neutral was done in the past, but the danger is if the ground system is not well connected, then you get a floating neutral*, or worse, a HOT chassis!

*and this can lead to the 120 items getting either low voltage or HIGH voltage, neither of which is good for the item.
Click to expand...

The floating neutral can happen even if you do the separate EGC as is required. The issue is when the neutral carries current there is a voltage drop on it. Connecting it to the non current carrying parts (for equipment grounding purposes) is still effective for helping to clear ground faults, and things do actually function, leaving false sense of "it works it must be ok". But that voltage drop does put same amount of rise on those non current carrying parts to true ground potential and increases the risk of shock. That rise is even more if there is a bad connection that adds resistance to the path and increases the shock risk even more, and of course totally open grounded conductor in the supply can put that chassis at full line to ground voltage.

This can also happen with separate EGC and grounded conductor, but will only happen during a ground fault condition coupled with a compromised EGC. Using the neutral for equipment grounding purposes can put a constant voltage rise on your non current carrying surfaces.
 
I need roughly 1kva......i'll probably drop in a small transformer.......but another question.......I understand the issue with the floating neutral......even if I dropped in a small transformer, I'd have one side grounded. I have the same issue in that case if the ground connection comes apart between the enclosure and the plug.
 
rswolff001 said:
I need roughly 1kva......i'll probably drop in a small transformer.......but another question.......I understand the issue with the floating neutral......even if I dropped in a small transformer, I'd have one side grounded. I have the same issue in that case if the ground connection comes apart between the enclosure and the plug.
Click to expand...
Yes, whether you use a transformer with a 120v or 120/240v secondary, one conductor should be grounded, as you would a service.

Yes, if the plug falls out, you'll lose the EGC, but that should happen last since most plug types have an EGC pin/prong that is longer.


Maybe I missed your question. Are you asking what if your bond breaks? The same concern: energized equipment and enclosures.
 
rswolff001 said:
I need roughly 1kva......i'll probably drop in a small transformer.......but another question.......I understand the issue with the floating neutral......even if I dropped in a small transformer, I'd have one side grounded. I have the same issue in that case if the ground connection comes apart between the enclosure and the plug.
Click to expand...
You are grounding the secondary side of that transformer so one leg will be at the same potential as ground. I don't see how that would be the same issue when you put a load on the ground. The primary would be fed by 2 phase conductors, right?
 
rswolff001 said:
I'd have one side grounded. I have the same issue in that case if the ground connection comes apart between the enclosure and the plug.
Click to expand...

When you ground a transformer, you are connecting _one_ terminal to 'ground'. Say this connection is to the chassis of the skid, and then the ground connection between the plug and the skid comes apart, so that the skid is 'floating'. The transformer won't 'energize' the skid because you still have only _one_ transformer terminal connected to the skid. The grounded transformer won't create a voltage difference between the skid and other grounded metal.
 
rswolff001 said:
I need roughly 1kva......i'll probably drop in a small transformer.......but another question.......I understand the issue with the floating neutral......even if I dropped in a small transformer, I'd have one side grounded. I have the same issue in that case if the ground connection comes apart between the enclosure and the plug.
Click to expand...
The supply plug to the skid will still have open EGC if the plug is compromised, it will no longer have current on EGC during normal operation though, so it presents the same problem as if you had no neutral loads to begin with and had a ground fault on the skid.

By placing the transformer on the skid for the 120 volt loads, and bonding a conductor to the skid all grounded conductor current is returning to the transformer on the skid and not to the main supply source none of this current will be on the EGC of the supply cord. If there is a ground fault on the 120 volt separately derived system on that skid, the fault current is returning to the transformer on the skid as well and is not dependent on the EGC in the main supply cord.
 
Status
Not open for further replies.
Top