Step-Down Step-Up Transformer Neutral to Ground Bonding

[kwired]

I'm just being overly cautious I guess. And possibly I still don't understand.
This is my (incorrect?) thinking:
On the secondary side of each transformer, there's X1 and X4. You have to pick one as the neutral and ground it or you don't establish 120 VAC across X1 to X4.
However I don't think you have to do that on the H1 and H4 600 V side since you're not pulling a Load at that point. I drew Neutral to Ground bonding in Red because I don't think its needed.
How far out in left field am I?
View attachment 22994

Any transformer secondary has voltage between secondary leads. If you don't ground any of them you have an ungrounded system, which is permitted by code with some requirements to go with it, but grounding the secondary is usually preferred.

Any conductor can be grounded, from a physics perspective. NEC generally will want you to ground a point that gives you lower volts to ground when possible - that is why "neutral" conductors are usually grounded.

A two wire secondary (disregarding x2 and x3 if they are not brought out as a usable circuit conuctor) has no "neutral" and therefore it doesn't matter which one you ground, either way results in same voltage to ground from the ungrounded conductor.

 

[Mechanada]

The neutral should not be bonded to ground again ahead of the 3KVA transformer, since it's shown being bonded to ground after the 5KVA one that feeds it. Otherwise, some neutral current will be flowing through the equipment ground conductors.

Having a similar discussion with a colleague. Can you explain why this occurs?

 

[kwired]

Having a similar discussion with a colleague. Can you explain why this occurs?

Current takes all possible paths, we don't want EGC's carrying normal operating current we want them to only carry current during fault conditions. Allowing them to carry normal operating current results in potential rise in volts to ground on non current carrying metallic components that are connected to the EGC. Might get away with it with little or no problems in some instances, until the right path gets opened and then you may have a sudden significant rise and definite potential shock hazards associated with it.

 

[Mechanada]

Current takes all possible paths, we don't want EGC's carrying normal operating current we want them to only carry current during fault conditions. Allowing them to carry normal operating current results in potential rise in volts to ground on non current carrying metallic components that are connected to the EGC. Might get away with it with little or no problems in some instances, until the right path gets opened and then you may have a sudden significant rise and definite potential shock hazards associated with it.

Thanks kwired - I'm somewhat of a novice on this stuff so am hoping to understand the phenomenon more thoroughly.

Would the EGC carry current only in a phase imbalance situation? If so, why would it matter which transformer (or both) get's grounded?

 

[kwired]

In theory there is no neutral current if the "phases" are balanced, reality is if the load(s) is connected to the neutral it will stabilize voltage so the load sees nominal line to neutral volts even if the loads should vary for some reason. Also in this situation not often is even identical loads going to be perfectly balanced and there will be some current on neutral even if only a few mA. If you arrange things so that EGC (along with any other objects that contact the EGC) can make a parallel path with the grounded (neutral) conductor then those items will carry current that is in proportion to the resistance of that path as compared to all other paths that exist. Simple circuit with same size and length neutral and EGC, half the current flows on neutral and half flows on EGC, whether it be 10 mA or 1000 amps.

 

[mpoulton]

I think there's a generator interlock problem here. I can't see any way that generator connection can be properly interlocked with the system as drawn. Although the generator input may already be existing, this change to the system makes the risk higher because it'll require someone to lock out the main disconnect (back near the utility connection, away from the generator location) in order to power both transmitters with the generator.