Maxwell'sLaw
Member
- Location
- New York City
- Occupation
- New construction field electrician
Greetings everyone.Hope all is well on your side of the street.
I have a most interesting question which a friend of mine brought to me, and which I want to share via this platform.
Please take a look at it, and try to see if y'all can provide me with some feedback/insight into this query. I thank you in advance for your kind assistance.
Here's the situation:
A contractor is installing a piece of equipment, with the following peculiarities:
Main power equipment is supplied by a 3∅ with neutral and ground 400VAC/400A feed from a xtrnsformer. Control voltage in the machine is provided by a 400/230VAC transformer with a neutral. Both the main power and the control power xtrnsformer neutrals are tied in on the same neutral bus in the main cabinet.
Now the question is this:
Wouldn't there be a voltage potential difference between the two xtrnsformers?
My friend, whom is also an industrial electrician has never seen that done before in that manner and is very curious.
My take on it is as follows:
From a perspective of a purely resistive load, I would say that his assessment is on the money, yet there may be other elements within that circuit which may indeed invalidate said assessment.
For example we need to consider if that circuit has any OCPD's, voltage regulators, or even perhaps any resistors between the two systems which may balance out that perceived potential difference.
He tells me there are no other circuits. Direct from the common 400V to the xtrnsformer and down to the common neutral bus. He suggested putting a diode on the 230V neutral to prevent power from going back up. Or at least putting in a separate neutral bar on the 230V side, which I know from my mechatronics training that the diode strategy is fundamentally the way to go, but another trades brother then asked him if it is a floating neutral, or are the neutrals bonded to equipment.
The friend requesting assistance informed us that the control voltage xtrnsformer on the secondaries does have a ground and the neutral bonded together in the xtrsformer, then coming out and going to a common neutral bus in the main cabinet.
That makes the potential on the neutrals the exact same, in other words: no potential difference in the two neutrals derived from two separate xformers going to a common bus after the bond.
I might be beating on an already dead horse, but:
a) would such a wiring setup require isolated grounds for each of the xtrnsformers, or would y'all say that a shared ground tied in to the bonding system satisfies proper wiring methodology?
* since the power from the control xtrnsfrmr is a separate system, I personally would prefer isolated grounds for each;
b) the way this system is described places it under the category of "separately derived systems" of the NEC. True or not?
Side Notes:
a) One of my two friends during the discussion offered this comment:
"I have seen ungrounded control transformers, aka a floating neutral and I have seen them bonded. The whole neutrals are only to be bonded at the main disconnect thing might be gone out the window since a transformer kind of is a new power source being that there is no physical connection between the primary and the secondary windings. So, in my opinion, right or wrong, the control power from a control transformer, is a new separate system. I don't think it's against any rules for the bonding of the neutrals to ground, especially if the equipment has its own grounding rod.
The only issue I have with floating neutrals is that it makes a green electrician scratch his head. If ya have 120/240 volt control power on a "floating" system. You may read 57 volts one leg to ground, and 63 volts on the other leg to ground.";
b) I've been told that the 400v main power feeding into an MCC, which is feeding the control transformer is built into the cabinet, and apparently is from the same power source (not a separate service);
c) I don't know if the incoming power is a grounded delta and if the neutral is coming straight off the pole, or if it's an ungrounded delta that’s being transformed to wye at a local transformer at the equipment;
d) as per my friend, the control transformer is inside the cabinet.
Can anyone within the forum offer any clarity on this topic?
I have a most interesting question which a friend of mine brought to me, and which I want to share via this platform.
Please take a look at it, and try to see if y'all can provide me with some feedback/insight into this query. I thank you in advance for your kind assistance.
Here's the situation:
A contractor is installing a piece of equipment, with the following peculiarities:
Main power equipment is supplied by a 3∅ with neutral and ground 400VAC/400A feed from a xtrnsformer. Control voltage in the machine is provided by a 400/230VAC transformer with a neutral. Both the main power and the control power xtrnsformer neutrals are tied in on the same neutral bus in the main cabinet.
Now the question is this:
Wouldn't there be a voltage potential difference between the two xtrnsformers?
My friend, whom is also an industrial electrician has never seen that done before in that manner and is very curious.
My take on it is as follows:
From a perspective of a purely resistive load, I would say that his assessment is on the money, yet there may be other elements within that circuit which may indeed invalidate said assessment.
For example we need to consider if that circuit has any OCPD's, voltage regulators, or even perhaps any resistors between the two systems which may balance out that perceived potential difference.
He tells me there are no other circuits. Direct from the common 400V to the xtrnsformer and down to the common neutral bus. He suggested putting a diode on the 230V neutral to prevent power from going back up. Or at least putting in a separate neutral bar on the 230V side, which I know from my mechatronics training that the diode strategy is fundamentally the way to go, but another trades brother then asked him if it is a floating neutral, or are the neutrals bonded to equipment.
The friend requesting assistance informed us that the control voltage xtrnsformer on the secondaries does have a ground and the neutral bonded together in the xtrsformer, then coming out and going to a common neutral bus in the main cabinet.
That makes the potential on the neutrals the exact same, in other words: no potential difference in the two neutrals derived from two separate xformers going to a common bus after the bond.
I might be beating on an already dead horse, but:
a) would such a wiring setup require isolated grounds for each of the xtrnsformers, or would y'all say that a shared ground tied in to the bonding system satisfies proper wiring methodology?
* since the power from the control xtrnsfrmr is a separate system, I personally would prefer isolated grounds for each;
b) the way this system is described places it under the category of "separately derived systems" of the NEC. True or not?
Side Notes:
a) One of my two friends during the discussion offered this comment:
"I have seen ungrounded control transformers, aka a floating neutral and I have seen them bonded. The whole neutrals are only to be bonded at the main disconnect thing might be gone out the window since a transformer kind of is a new power source being that there is no physical connection between the primary and the secondary windings. So, in my opinion, right or wrong, the control power from a control transformer, is a new separate system. I don't think it's against any rules for the bonding of the neutrals to ground, especially if the equipment has its own grounding rod.
The only issue I have with floating neutrals is that it makes a green electrician scratch his head. If ya have 120/240 volt control power on a "floating" system. You may read 57 volts one leg to ground, and 63 volts on the other leg to ground.";
b) I've been told that the 400v main power feeding into an MCC, which is feeding the control transformer is built into the cabinet, and apparently is from the same power source (not a separate service);
c) I don't know if the incoming power is a grounded delta and if the neutral is coming straight off the pole, or if it's an ungrounded delta that’s being transformed to wye at a local transformer at the equipment;
d) as per my friend, the control transformer is inside the cabinet.
Can anyone within the forum offer any clarity on this topic?
