High Resistance Grounding for 1000 volts and below

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What is the voltage limit for High Resistance Grounding? Will the code allow High Resistance Grounding for 240Y or 460Y voltage system?
 
"Do you know the reason why it is not allowed for voltages lower than 480 volts or at 240 volts?

How will it be unsafe if HRG is applied to lower than 480 volts?
 
Based on IEEE 142 explanations about HRG, there is no implied reason that it can not be used for voltages lower than 480 volts. How come NEC is limiting its use to 480 volts and above only? Should they consider revising the code to allow lower than 480 volts? Most Transformers are rated 480 volts but nominal voltage used is 460volts. Why will then NEC limit the voltage to only 480 volts? Does it mean that industrial customers needs to increase first their voltage to 480 volts inorder to use the benefits of HRG?

If HRG is far better than an ungrounded system and ungrounded system can be used for 240 volts, how come NEC is limiting its application to only 480volts and above?
 
Solidly grounded low-voltage systems of less than 150 V do not pose any major difficulties in protection because either the arc voltage will be greater then the voltage source and self extinguish, or the fault will weld at the point of fault causing overcurrent protection to operate.

The nominal voltage referenced would be 480V, motors may have 460V rating, but the standard system voltage is 480V. It would be rather odd to have a U.S. design that was not 480V. Certainly you will find 440V, 410V, 380V, etc. in other countries, but the NEC is not enforceable outside the U.S.

The use is permitted based on meeting the conditions. Not all facilities will be able to meet those conditions. Also, no reference is made to applications above 1000V, and therefore you can apply HRG to medium voltage systems and above without being in violation of the NEC.
 
"The nominal voltage referenced would be 480V, motors may have 460V rating, but the standard system voltage is 480V. It would be rather odd to have a U.S. design that was not 480V. Certainly you will find 440V, 410V, 380V, etc. in other countries, but the NEC is not enforceable outside the U.S. "

The problem lies in what the code says that it is only permitted from 480 volts to 1000 volts. In the ungrounded system which I quote

" Alternating current Systems of 50 volts to 1000 volts Not Required to Be Grounded. The following ac systems of 50 volts to 1000 volts shall be permitted to be grounded BUT SHALL NOT BE REQUIRED TO BE GROUNDED"

Why will the code allow ungrounded for 50 volts to 1000 volts while limit HRG to 480 to 1000 volts. Isn't it that HRG is safer than Ungrounded system based on IEEE 142-1991?
 
poweringtech said:
Any expert here on system grounding?
Yep, there are a bunch. I'm not one, cause I know I don't understand what the code panels think they are saving us from.

poweringtech said:
...Will the code allow High Resistance Grounding for 240Y ...
I'v never seen 240Y. Did you mean 208Y? If you did, as has already been said, it would have to be grounded (250.20B1)

poweringtech said:
...How will it be unsafe if HRG is applied to lower than 480 volts?...
I have no idea - and I've looked plenty.

GeorgeS said:
...Originally Posted by poweringtech: ...ungrounded system can be used for 240 volts...

I don't believe that to be correct, according to 250.20(B)(1), but am not certain of that....
George - 240 is always Delta. There is no way to ground it and have the ungrounded conductors 150V or less. (Possible oddball exceptions noted.) So, 240D is exempt from 250.20B1. Now if it were 240/120D, then 250.20B3 would apply, and it would have to be grounded.

GeorgeS said:
...Impedance Grounded Systems are permitted for voltages between 480V and 1000V. See 250.36....
And for 1000V and up see 250.186

poweringtech said:
...Why will the code allow ungrounded for 50 volts to 1000 volts while limit HRG to 480 to 1000 volts. Isn't it that HRG is safer than Ungrounded system based on IEEE 142-1991?....
Pretty well asked and answered. Personally, I really don't know.

But I can tell you the code does like solidly grounded systems (ie. think they are safer) better than IG systems. That one baffles me too.

carl
 
coulter said:
George - 240 is always Delta. There is no way to ground it and have the ungrounded conductors 150V or less.
The typical configuration is midpoint grounded, which puts the L-N voltage (the voltage referenced in 250.20(B)(1)) at 120V. :)

Edit: Actually, it looks (on further review) as though (B)(1) is intended for single phase, (B)(2) for 120/208 three phase, and (B)(3) for 120/240 midpoint grounded delta three phase. Agreed, Carl?

Tech, I'm no expert.

Perhaps you have a design in progress that would benefit, and that's what's bringing the question up for you? Perhaps if you lay the parameters out on the table, the pros and cons (or potential code violations) could be considered by your peers in engineering? :)
 
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georgestolz said:
The typical configuration is midpoint grounded, which puts the L-N voltage (the voltage referenced in 250.20(B)(1)) at 120V. :)

Edit: Actually, it looks (on further review) as though (B)(1) is intended for single phase, (B)(2) for 120/208 three phase, and (B)(3) for 120/240 midpoint grounded delta three phase. Agreed, Carl?

A laugh on me:) I never even considered single phase. The OP started with 3ph and I likely do one or less single phase ckts a year. Single Phase? That's what they hook up to houses isn't it?;)

We're close. I would add 480Y to B2. Also, I would suggest that B1, in addition to requiring house standard 120/240, 1ph to be grounded, also allows ungrounded 240D and 480D.

carl
 
While standard design for 240 volts system is DELTA, designer in the future may insist on 240Y/139 volts system using 240 volts 3 phase and 240 volts single phase line-to-line.

DELTA may converted to a HRG by installing a grounding tranformer or zigzag transformer and connecting a resistor to convert them to HRG.
 
poweringtech said:
... designer in the future may insist on 240Y/139 volts system using 240 volts 3 phase and 240 volts single phase line-to-line. ...
That's true. I believe I have even seen transformers cataloged with that connection. I've never seen one in use. What do you think would be the design drivers behind specifying this?

poweringtech said:
....DELTA may converted to a HRG by installing a grounding tranformer or zigzag transformer and connecting a resistor to convert them to HRG.
I've seen pictures of this in books as well, but again, never seen one installed. Again, why?

carl
 
coulter said:
That's true. I believe I have even seen transformers cataloged with that connection. I've never seen one in use. What do you think would be the design drivers behind specifying this?
carl

The most current typical use of 240Y transformers is for isolating AC/DC drive systems. Because they are typically specified along with motors drive isolation transformers are usually cataloged with utilization instead of nominal voltages, so you find 460-460Y/266 and 460-230Y/133 as well as "strange" kVA ratings.
 
Jim -

Yes that true. I've seen a 460Y. I haven't seen a 240Y, but I've heard of them in conjunction with DIT. Always figured it was the result of a screwed up initial design. What would be the design driver for using 240 over 480? I don't think the semiconductors are any cheaper - motors aren't any cheaper.

I reviewed your post, maybe you were thinking of a DC drive, there 240 would almost make sense. But, I'm weak on DC drives - you could tell me different and I'd believe you.

carl
 
Some possible reasons:
If your service was 240V 3PH and you wanted a DIT then you would need a 240Y unit. If your equipment came from overseas it might be rated for 240V only (why people don't specify voltage when they order special equipment is beyond me).

I have just finished some systems studies at three different paper mills and they all had a mix of 480Y and 240Y DITs.
 
jim dungar said:
...If your service was 240V 3PH and you wanted a DIT then you would need a 240Y unit. ...
Or, order a 240D/480Y DIT (or 460Y).

jim dungar said:
...If your equipment came from overseas it might be rated for 240V only (why people don't specify voltage when they order special equipment is beyond me)....
I'd easily agree with the second half of that.

jim dungar said:
...I have just finished some systems studies at three different paper mills and they all had a mix of 480Y and 240Y DITs.
Having worked in a papermill, I wouldn't find that odd at all.;)

carl
 
In the Maryland suburbs outside of Washington DC the standard voltage is 460 as supplied by the utility, The number of PQ calls I recieve for low voltage for the 120/208 VAC distribution. I tell them (electrical contractors or facility folks) to arrange an outage to change the taps to raise the secondary voltage. I am surprised by the number of electricians that are surprised by the 460 and the idea that they can change transformer taps to overcome this. The utility tells me they do not have taps on their distribution transformers at the customers connections, and even if they did they would prefer not to change the taps for a single customer. 460 VAC primary transformers are available but most purchasing agents buy the same transformers they buy for Washington DC.

I have also seen 240/139 VAC wye transformers, this was for a special installation, all the equipment was 240 VAC 3 phase, the customer supplied the transformers, there were no phase to neutral connection loads.
 
brian john said:
In the Maryland suburbs outside of Washington DC the standard voltage is 460 as supplied by the utility

That is too strange! What is the utility's distribution primary voltage? That might have something to do with why their secondary is 460 V coming out of the transformer. Does the transformer nameplate actually say 460 V for rated secondary voltage, or is the 460 V a result of using a transformer fed from a primary voltage that is lower than what is on the transformer nameplate for rated primary voltage?
 
The issue in this thread is why is it not permitted in the code to use High Resistance grounded in voltages lower than 480 volts?

What if the customer decides to use HRG because he wants continuity of service even with a single line to ground fault at 240 volts three phase?

Any code expert or member of the code who can explain? If it is safe to use HRG at lower than 480 volts, should the code be changed to allow HRG be applied to 240 volts?
 
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