Medium voltage vs low voltage

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LMAO

Senior Member
Location
Texas
I apologize if this is not the correct forum. I am looking for general design consideration differences between medium voltage and low voltage. I do know the difference between the two: LV<600V, 600V<MV<69kV. I just like to know, generally speaking, how they are different. For example NEC covers low voltage; what is the equivalent of NEC for medium voltage?

From an engineer point of view, what are the other "main" differences between the two?

Thanks
 

Fulthrotl

~Autocorrect is My Worst Enema.~
I apologize if this is not the correct forum. I am looking for general design consideration differences between medium voltage and low voltage. I do know the difference between the two: LV<600V, 600V<MV<69kV. I just like to know, generally speaking, how they are different. For example NEC covers low voltage; what is the equivalent of NEC for medium voltage?

From an engineer point of view, what are the other "main" differences between the two?

Thanks

the = for MV is whatever the AHJ says it is.
 

ATSman

ATSman
Location
San Francisco Bay Area
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Electrical Engineer/ Electrical Testing & Controls
Generally speaking based on what I've seen, the two main reasons MV is used over LV is:
1. Whenever large amounts of power (KVA) is required to supply the load over
large areas and distances (such as a campus of multiple bldgs.) it is less costly
and takes up less space (less copper.)
2. Voltage drop considerations.
I am sure someone can come up with others reasons as well.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
The NEC covers MV, but not when it is utility equipment.
Utilities usually voluntarily follow NESC, but customize their own policy.

Sent from my XT1585 using Tapatalk
 

Jraef

Moderator, OTD
Staff member
Location
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Electrical Engineer
I apologize if this is not the correct forum. I am looking for general design consideration differences between medium voltage and low voltage. I do know the difference between the two: LV<600V, 600V<MV<69kV. I just like to know, generally speaking, how they are different. For example NEC covers low voltage; what is the equivalent of NEC for medium voltage?

From an engineer point of view, what are the other "main" differences between the two?

Thanks
Slight corrections:
ANSI /IEEE definitions:
LV = < 1000V
MV = > 1000V to 25kV
HV = > 25kV

NEC covers LV and MV but defines LV as <= 600V. Outside of North America there are pockets of 690V, especially in maritime industries, but generally the NEC doesn't apply to those places. Solar has been pushing that envelope though and I expect the NEC to shift the LV definition to match ANSI/IEEE.
 

Sahib

Senior Member
Location
India
From an engineer point of view, what are the other "main" differences between the two?

Thanks
One main difference between the two, IMO, is the length of arc they may produce. I think when voltage is greater than 1kV, the arc length can be much longer.
 

Sahib

Senior Member
Location
India
My difficulty is arc flash hazard can be much greater in a LV system than that due to MV system even though the latter is capable of producing much longer arc........... The operation of MV protective gear must be much faster to prevent that from happening.:)
 

Tony S

Senior Member
LV can be more destructive as modern switchgear design seem to have been cut to the bone, they are just about adequate for the task of in hand.

MV switchgear does tend to be more robust. After 40+ years in the trade and I've only seen one instance of flash over in MV switchgear. It was caused by a mouse somehow getting in to the busbar spouts, it created a bit of mess a and lost production.
 

Besoeker

Senior Member
Location
UK
LV can be more destructive as modern switchgear design seem to have been cut to the bone, they are just about adequate for the task of in hand.

MV switchgear does tend to be more robust. After 40+ years in the trade and I've only seen one instance of flash over in MV switchgear. It was caused by a mouse somehow getting in to the busbar spouts, it created a bit of mess a and lost production.

Yes, I've seen one instance of that too. It was in a paper mill and, for reasons I could never fathom, the buss bar chamber was close to floor level rather than the conventional arrangement along the top. And yes, a bit of a mess. The speculation is that the offending piece(s) of vermin was a rat although there wasn't enough of it left to tell.
 

GoldDigger

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On the Stanford University campus I saw the aftermath of a squirrel climbing into a roll-out MV breaker section at an outdoor substation. The remnants of the animal was squirrel sized, but you could not be sure from just the tail bones that it was not a large rat.
It took about a full day before they had all of the carbon cleaned out of the switchgear and had the sub back in full operation.
 

Besoeker

Senior Member
Location
UK
On the Stanford University campus I saw the aftermath of a squirrel climbing into a roll-out MV breaker section at an outdoor substation. The remnants of the animal was squirrel sized, but you could not be sure from just the tail bones that it was not a large rat.
It took about a full day before they had all of the carbon cleaned out of the switchgear and had the sub back in full operation.
Not a pleasant clean-up job........
 

Tony S

Senior Member
On the Stanford University campus I saw the aftermath of a squirrel climbing into a roll-out MV breaker section at an outdoor substation. The remnants of the animal was squirrel sized, but you could not be sure from just the tail bones that it was not a large rat.
It took about a full day before they had all of the carbon cleaned out of the switchgear and had the sub back in full operation.

GEC came out to take photographs of the carnage caused by our little fury friend. By the time they arrived I'd given our friend a christian burial.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Slight corrections:
ANSI /IEEE definitions:
LV = < 1000V
MV = > 1000V to 25kV
HV = > 25kV

NEC covers LV and MV but defines LV as <= 600V. Outside of North America there are pockets of 690V, especially in maritime industries, but generally the NEC doesn't apply to those places. Solar has been pushing that envelope though and I expect the NEC to shift the LV definition to match ANSI/IEEE.
Starting with the 2014 code and continuing into the 2017 NEC, most of the references to 600 volts or less have been replaced with 1000 volts or less.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
My difficulty is arc flash hazard can be much greater in a LV system than that due to MV system even though the latter is capable of producing much longer arc........... The operation of MV protective gear must be much faster to prevent that from happening.:)

As a utility guy, I've seen more squirrel suicides than I can count in outdoor substations, mostly 12,470/7200. Usually the squirrel is vaporized, but little damage is done to the associated conductors or buswork, even if the fault becomes phase to phase. I think the difference in fault damage is probably due the fact than LV fault protection usually involves thermal/magnetic breakers, which have a fairly long clearing time unless it's a bolted fault. MV protection, on the other hand, usually involves CT supplied relaying or fuses. Likely faster clearing times. As far as physical differences, it's all about insulation values. Size of insulators (distance to grounds), wiring considerations (such as cabling construction, stress cones at terminations, etc). Explosions from arc faults are related to power and time and available metal, not just fault current. A MV fault may have more fault current than a LV fault, but there's likely much more metal associated with LV conductors and switchgear. I've seen 277/480V faults that are way more scary than 7200 faults. I'm sure there's more to it, but I'd rather be close to a MV distribution fault in open air than inside a switchgear with a 277/480V fault. Just me, though.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Sorry about the "mission creep" from design considerations to arc faults.......

As far as design, it all depends on cost, components availability, distance of source to load, physical size, etc. Nuclear plants are a good example. Small motors (up to 500HP) are usually 480V, larger motors (up to 5000 HP) are usually 4160V and the big boys (up to 12,000 HP) are usually 13,800 or 24,000V. Generator outputs in the gigawatt range are mostly 25,000V. Transmission out to the distribution substations is anywhere from 60KV to 345KV AC and 500KV to 1MV DC. So, your question is not a one line answer. That's why engineers need college degrees. NEC can't even begin to cover it all.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
MV is superior in many ways, so much show its increasingly used in data centers. The only reasons why 480 volts remains popular is because it does not require the same level of training as LV and its what building maintenance/owners feel the most comfortable having.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Sorry about the "mission creep" from design considerations to arc faults.......

As far as design, it all depends on cost, components availability, distance of source to load, physical size, etc. Nuclear plants are a good example. Small motors (up to 500HP) are usually 480V, larger motors (up to 5000 HP) are usually 4160V and the big boys (up to 12,000 HP) are usually 13,800 or 24,000V. Generator outputs in the gigawatt range are mostly 25,000V. Transmission out to the distribution substations is anywhere from 60KV to 345KV AC and 500KV to 1MV DC. So, your question is not a one line answer. That's why engineers need college degrees. NEC can't even begin to cover it all.

There are things a college does not teach though. Experience from older, more experienced guys is one of them. Also, be it MV or LV, there is often so much to consider that most projects of such scope take a team of people all with varying degrees to design and execute. I can picture a PhD asking the same question as the OP.

While for a data center, here is one comparison:


http://www.ccgfacilities.com/pdf/MEDIUM VOLTAGE ELECTRICAL SYSTEMS FOR DATA CENTERS.pdf
 
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