XO

[elrod96]

I need to set a 3 phase 480v 7.5 kva transformer just to feed a conveyor control panel at 240v. Do I still ground XO even with a foreign drive? My concern is the resistance on the XO terminal is very low to grnd and I'm afraid of a dead short or even hurting the Japanese drive. Any expert advice?


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[dionysius]

I need to set a 3 phase 480v 7.5 kva transformer just to feed a conveyor control panel at 240v. Do I still ground XO even with a foreign drive? My concern is the resistance on the XO terminal is very low to grnd and I'm afraid of a dead short or even hurting the Japanese drive. Any expert advice?


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How about calling the manufacturer??? Is this a Delta to Wye trafo. You are deriving a separate service. XO is normally grounded in such case IMHO.

 

[Smart $]

What info is on the conveyor nameplate?

By Japanese drive, do you mean the motor controller is a VFD? If yes, make, model, PN, and/or link to documentation...

As a reminder, if you do not ground the secondary, a ground detector is required. See 250.21(A).

 

[elrod96]

Yes sir. Delta to wye. It's used and if I'm not mistaken, it has been discontinued. Everywhere I look in the codebook just talks about grounding XO when dealing with lighting panels or similar distribution panels. In my case I'm just dealing with a control conveyor. And it seems to be running perfectly without XO grounded. I have read both ways to where grounding XO can hurt foreign inverters but not grounding XO could potentially put someone at risk for shock. With XO reading so little resistance to ground it really makes me feel like I will be completing a dead short. The Ohmic value is less than 2 ohms.


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[elrod96]

It is also 230 est where I am so I can include documentation in about 6 or 7 hrs. Thank you so much for the quick responses!!


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[augie47]

I'm a bit confused. If the system is delta to wye I would think your voltage would be 208/120 in which case 250.20 would require XO to be bonded.
Where the system can be grounded so that the maximumvoltage to ground on the ungrounded conductorsdoes not exceed 150 volt

If you actually have 240, I would think it was a delta-delta transformer with a 240/120 secondary and 250.20 does not require the system to be grounded.
Where the system is 3-phase, 4-wire, delta connected inwhich the midpoint of one phase winding is used as acircuit conductor

but, as mentioned above, if you dont ground it a detector is needed.

I would first check with the manufacturer to see if they have a requirement.

 

[kwired]

If it is a wye secondary the neutral is usually designated with X0.

If it is a delta secondary with a midpoint tap for one phase - that tap is usually designated with X4.

If it is wye it is most likely 208/120, is possible to be 240/139 but is a kind of rare to find such a thing.

Your drive probably will not care if the source is grounded or not. Code will still want either a grounded system, high impedance grounding (typically with wye systems) or non grounded (typically with delta systems), both of the latter need ground fault detection/indication systems if used.

One common issue with drives is surge protection they incorporate on the input side of the drive. Most of your off the shelf drives have MOV's connected each input line to ground - if the voltage to ground exceeds the MOV rating they will fail in short time. Corner ground systems will often be an issue because of the full line to line voltage to ground will exceed the MOV ratings. Ungrounded systems can experience low or high voltage to ground depending on conditions. Grounded systems have stable input voltages in reference to ground, and grounded wye systems will give you approximately equal voltage to ground on each leg.

 

[ActionDave]

I'm a bit confused. If the system is delta to wye I would think your voltage would be 208/120 in which case 250.20 would require XO to be bonded.
Where the system can be grounded so that the maximumvoltage to ground on the ungrounded conductorsdoes not exceed 150 volt

If you actually have 240, I would think it was a delta-delta transformer with a 240/120 secondary and 250.20 does not require the system to be grounded.
Where the system is 3-phase, 4-wire, delta connected inwhich the midpoint of one phase winding is used as acircuit conductor

but, as mentioned above, if you dont ground it a detector is needed.

I would first check with the manufacturer to see if they have a requirement.

Would 250.21(3) apply for the op?

 

[winnie]

If you are already reading low resistance between X0 and ground, then the system may already be grounded (intentionally or through a fault) somewhere.

A solidly grounded system should have _one_ and only _one_ connection to ground, generally the transformer X0 although there are other options. Multiple ground connections can either cause 'objectionable current' or a full blown 'short circuit'.

IMHO you should investigate why you have such a low reading to ground, and if this is a fault fix it and then solidly ground the transformer.

-Jon

 

[Jraef]

If you are already reading low resistance between X0 and ground, then the system may already be grounded (intentionally or through a fault) somewhere.

A solidly grounded system should have _one_ and only _one_ connection to ground, generally the transformer X0 although there are other options. Multiple ground connections can either cause 'objectionable current' or a full blown 'short circuit'.

IMHO you should investigate why you have such a low reading to ground, and if this is a fault fix it and then solidly ground the transformer.

-Jon

Most likely the drive has a set of MOVs (surge arrestees) across the diode bridge input that are tied to a ground reference. Many drive mfrs give you specific instructions that if connected to a delta source, that ground reference connection must be removed, otherwise it attempts, very briefly, to become the Y point of the secondary when there is a ground fault somewhere, until the MOVs vaporize and damage other things. Japanese drive mfrs however tend to ignore or are unconcerned with the consequences of this in my observation.

Side note: isn't there an exception somewhere to the ground monitoring requirement if the transformer is used as a voltage change for one specific machine? The point was that the ground monitoring was required for a distributed system based on it being a separately derived source. It still technically is when used this way, but the exception was allowed because it was dedicated to that one machine. I might be wrong, I don't have access to my code docs today, or the exception might be related to it being in the same enclosure as the motor controller, something like that. Maybe someone else can thake a quick look.

 

[ActionDave]

Most likely the drive has a set of MOVs (surge arrestees) across the diode bridge input that are tied to a ground reference. Many drive mfrs give you specific instructions that if connected to a delta source, that ground reference connection must be removed, otherwise it attempts, very briefly, to become the Y point of the secondary when there is a ground fault somewhere, until the MOVs vaporize and damage other things. Japanese drive mfrs however tend to ignore or are unconcerned with the consequences of this in my observation.

Side note: isn't there an exception somewhere to the ground monitoring requirement if the transformer is used as a voltage change for one specific machine? The point was that the ground monitoring was required for a distributed system based on it being a separately derived source. It still technically is when used this way, but the exception was allowed because it was dedicated to that one machine. I might be wrong, I don't have access to my code docs today, or the exception might be related to it being in the same enclosure as the motor controller, something like that. Maybe someone else can thake a quick look.

This one maybe?

250.21 The following ac systems of 50 volts to 1000
volts shall be permitted to be grounded but shall not be
required to be grounded:

(3) Separately derived systems supplied by transformers
that have a primary voltage rating of 1000 volts or less,
provided that all the following conditions are met:
a. The system is used exclusively for control circuits.
b. The conditions of maintenance and supervision ensure
that only qualified persons service the installation.
c. Continuity of control power is required.

 

[Smart $]

This one maybe?

A motor drive supply can hardly be considered control circuits in many circles... and I doubt it will in this one. But as food for thought, this one definitely seems counterintuitive given that a VFD "controls" the connected motor, and thus does so with a control circuit.

:slaphead:

That said, I believe this implementation better aligns with 250.21(A) "(2) Separately derived systems used exclusively for rectifiers that supply only adjustable-speed industrial drives". It's a bit dated, from back when ASD's were installed in parts rather than as one integral unit as VFD's now are.

Nonetheless, a ground detector is still required per 250.21(B)(1).

 

[Jraef]

A motor drive supply can hardly be considered control circuits in many circles... and I doubt it will in this one. But as food for thought, this one definitely seems counterintuitive given that a VFD "controls" the connected motor, and thus does so with a control circuit.

:slaphead:

That said, I believe this implementation better aligns with 250.21(A) "(2) Separately derived systems used exclusively for rectifiers that supply only adjustable-speed industrial drives". It's a bit dated, from back when ASD's were installed in parts rather than as one integral unit as VFD's now are.

Nonetheless, a ground detector is still required per 250.21(B)(1).

Yep, that's what I was thinking of but you're right. No need to ground it, but you still need the monitoring.