I'm in charge of distribution of this condo project. We have a 600/347V main service. I just installed a 600v - 208/120v transformer and everything is groovy. The star point (X0) on the secondary side is grounded with uninsulated copper wire which by code it should be. I just measured the ground wire and it is carrying 10 amps. I know that this must be the unbalanced neutral current, but is this ok for a ground wire to carry that much current uninsulated? Is this normal? I didn't think that ground wires are supposed to carry current normally (only under fault conditions).
*Where* are you measuring 10 amps? Between the load and the transformer star point, or in the grounding conductor connecting the star point to ground? If you have a little unbalanced current in the neutral, that's normal - depending on the size of the service a 10 amp unbalance may be very good. But if you've got 10 amps circulating in the *grounding* conductor, there's something else wrong in the system - at least an extra connection in the neutral out in the wiring system.
The bare ground you have is carrying the current on it to ground. And will take the path of least resistance... the copper wire to ground...
if you touch it, with your feet wet and grounded you might share the load, but only in direct proportion to the resistance of your body, many thousand ohms.. to the hopefully big fat copper wire... virtually zero ohms...takes all of the current... unless :)) the ground connection or conductivity goes bad in dry rocky soil or whatever (seasonally)... and then the current goes though your body to ground.
its a good question...I wonder about things like that myself...what if a transformer coil shorted internally.. that could be a real lively ground.
What ~are the advantages in not insulating the ground anyway? Better contact through the conduit possibly, creating a continuous ground even if the conduit separates or corrodes... is probably the reason..same logic in the distributed system. It is a ground after all...so it is inherently safe to that degree.
If the ground connection fails though it can be a problem in the case of a short. Notice that grounded neutrals are insulated.
Have you seen the latest on 'arc flash explosion'... you are doing a large system, its an issue... there have been some good references posted on the NG... recently.
Proximity of the step down transformer to the 600vac mains, and the distance or length of wire to the first 110 outlets etc is an issue along with the size of the transformer in bolted short conditions and wire size and interrupt ratings on the utility service and your own transformer... thats all changing as utility companies are doubling up on the service transformers so that you have 5x the capacity you need most of the time... a dead short can be devastatingly explosive in these conditions as the delay time to trip can be excessive.
Interestingly, a *less than dead bolted short, say an 60% dead short depending on the current flow and interrupt device, could extend the interrupt time to seconds while one and all fry like chickens... so its tricky business..especially as you get into higher voltages closer to heavier feed lines.
The safe sizing for interupt devices provides excessive tripping on hot days with low voltage (runs up the amperage)... Heavier trip devices will not trip at all in some dangerous conditions.
Dividing the load to several instead of just one step down transformer will allow smaller and safer interupt devices... two instead of one for instance halves the size of the feed to each step down transformer..
Time to interrupt on your circuit breakers is important..and not easy to sort out beyond the stock offerings... while you will end up using standard items, the configuration is an issue at 600vac primary voltages... the resistance in the feeders and distance to the primary transformers etc.. You do know that you need a high voltage qualified meter to use at the mains on the 600vac of course. I forget the way its called out...category2 or 3 maybe.
To illustrate..say you wanted a 110 outlet next to the main service and transformer. You would do several things to make it safer... you would use a 10 or 15 amp breaker on that circuit, and #14awg wire...and you would not try to shorten the run, you might loop it up 8' and back down if it were to be used say on a computer or something a user could come into intimate contact with..or on a wet basement floor etc.. You would design resistance and amperage limitations into that circuit...you might even want to include a fast acting one time fuze box for it...or gfi etc to eliminate the time delay seen with breakers and dual element fuses.
in contrast 100' away..you run for the 110 you use , 20 amp breakers and size 12 wire usually.
right, that does make sense. I guess that's why they want that ground plate buried well.
In this case, the wire is free air.
This project has 5 transformers (600-208/120) spread throughout the building (and currently two 600v-480v for the cranes). The farthest being about 800' away. The closest is right beside the main service. I thought this is done more for voltage drop, but maybe the engineers are thinking about safety.
Yeah, I heard about meters blowing up in people's hands because of the voltage. I have some kind of Fluke that supposed to be rated for 750v but I never did check the category. Too late now, it works.
I've never heard or seen a 10amp breaker. (I live in Canada)
You can't run a 20 amp breaker on an ordinary outlet.
ps. Thanks for your email, I'm still digesting some of the theories.
The arc flash explosion issue has been around but only lately become a concern due to utility companies doubling up on the line transformers..so many electrical engineers are not aware of the issue..
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but I never did
'works' is good Fluke is probably fine...750vac rated proves it... but 'works' is not safe if the meter is rated for say 110 or 220 max
for a control job I am doing, for wire size issues I wanted to go to a 10amp breaker...I couldnt find a 10 I had to fit a 15 amp, and run stiff 14 wire, then when I got to the panel I fused it at 10 amps and used #16 and 18 machine tool wire. It serves a 24vac control transformer 75 va, a small PLC and two phone dialers. less than an amp at 110vac
amp
In the US 20 amps is more or less standard service for a string of 5 or 6 duplex outlets, and for most light circuits. Some notable exceptions for bath rooms, pools, etc.
theories.
The arc flash thang is not a theory these days but a growing problem... is there some scare factor in that? Probably...but its still an issue.
Shouldn't that be white, as it is actually the Neutral, not Ground?
Also. with the true ground, it may be called upon to take a lightning strike to ground potential. High frequency signals (like lightning) do not like insulation nor do they like conduit - they act as a choke to these signals.
If what you are talking about is about 10 amps of "leakage" in, what?, a 200 "service" it could be the sum of a lot of "minor" leakages. Note that a 10 amp fault to ground would not trip any breaker.
If you have the time, you can use your "Amprobe" to see if one or two circuits have significant leakage while most have next to none or whether everything "leaks." (Easy to do: just clamp around all the conductors (other than the green or bare one) going out of the box through one opening.
But that's why the grounding exists in the first place: to get that current back to where it belongs without making metal objects "hot."
(If it turns out that only one circuit has most of the leakage, it would be a good idea to take off some covers and check for crosses between the boxes and neutral wires and even a connection of a lighting circuit between HOT and GROUND.)
| I'm in charge of distribution of this condo project. We have a 600/347V main | service. | I just installed a 600v - 208/120v transformer and everything is groovy. | The star point (X0) on the secondary side is grounded with uninsulated | copper wire which by code it should be. | I just measured the ground wire and it is carrying 10 amps. | I know that this must be the unbalanced neutral current, but is this ok for | a ground wire to carry that much current uninsulated? | Is this normal? I didn't think that ground wires are supposed to carry | current normally (only under fault conditions).
Do you have 2 wires attached to the X0 star point? Is that serving as your neutral-to-ground bond? Which wire are you measuring current on? If the current is on the neutral going to the panels, then that is the unbalance current. But current on the ground bond indicates a problem. If the current is flowing in one direction from there, then you have to have current flowing the other way somewhere else to complete the circuit and balance the "network". What do you read when you clamp around all three primary (600v) wires together? What do you read when you clamp on all four secondary (208Y/120) wires together?
If it were possible to clamp around all 8 wires (3 primary, 4 secondary, and grounding) all at once, it should be zero. But if you got current there, then you have a core fault (leaking out through the frame) and a bad transformer.
hi(i am power engineer email snipped-for-privacy@yahoo.com)
------------------------------------------------- Answer: i think in grounded place voltage in ideal status is 0 (zero) so
10amp*0=0 Watt. But you have a limited ground and this current (10 amp) make this earth more dry in time-so you cant keep ohm in standard value and voltage became to be more than 0- so you have a losing power like (1v*10amp=10w) 10w for each 1 volt. you must to doing ( phase unbalancy calculation ) and solve this problem. So you can make a secondry circuit in lov v side for keep this leckage in less condition. follow this topic and send answer to site.
hi(i am power engineer email snipped-for-privacy@yahoo.com)
------------------------------------------------- Answer: i think in grounded place voltage in ideal status is 0 (zero) so
10amp*0=0 Watt. But you have a limited ground and this current (10 amp) make this earth more dry in time-so you cant keep ohm in standard value and voltage became to be more than 0- so you have a losing power like (1v*10amp=10w) 10w for each 1 volt. you must to doing ( phase unbalancy calculation ) and solve this problem. So you can make a secondry circuit in lov v side for keep this leckage in less condition. follow this topic and send answer to site.
Absolutly not. Ground is not suposed to carry any current except milliamps. You have a ground fault. It must be located and eliminated. Note, a ground fault is not an unbalanced neutral current which flows in the neutral wire not the bare or green wire, ground or to any grounding stakes, metal chassis or enclosures. Bob
Some About unbalancy: emagine that we have a transformer 1250 KVA We allow 10% unbalancy acc. to design criteria. so: 1250 * 10% = 125 KVA if in secondry side we use 400 v then: s=rad(3)*V*I > I=125000/(1.7*400)=183.8 A That means your current in one phase can be 183A more than another phase.
I've been busy with other parts of the project and I finally had time to recheck this ground thing. The transformer is hooked directly up to one panel. This panel is feeding temporary panels around the job site including some permanent circuits. I checked all of the ground wires (fortunately this is a concrete building and we are using coreline - not emt, so all outgoing conduits have ground wires). One ground has current on it. It was hooked up to a temp panel that some other electrician hooked up. He didn't take the neutral bond screw out and that was the whole problem.
Thanks to all the answered. This problem had lead to some more questions that I'll be posting at a later time.
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