Load Break Elbows

Status
Not open for further replies.
M

msteiner

Senior Member
Location
Maryland
I'm reviewing a submittal for load break elbows on a 4160V system. The contractor is proposing 200A elbows, but the upstream protection is 250A fuses. Any concerns with using these elbows?
 
Possibly depends on what code applies (is it for utility or for NEC application) as well as what the expected load is?
 
msteiner said:
The NEC applies. The load being fed is a 750kVA xfmr (not fully loaded).
Click to expand...
I honestly don't know a lot of these kind of finer details on medium voltage equipment, but since it is load break equipment I would guess there is an interrupt rating associated with it and it is possibly fine with the higher overcurrent protection as long as it doesn't carry more load than rated for and interrupt rating isn't exceeded? By interrupt rating I mean not just shot circuit/ground fault but also regular switching rating.
 
Interesting question, and a bit tricky as well. I understand that there are only two ratings for these elbows: 200 amp "load break" and 600 amp "dead break" (I might be wrong about that). The difference is whether they can be safely disconnected while current is flowing through them. What I don't know is whether the 200 amp rating means that is the most current they can safely be carrying when they are disconnected, or whether that is more like an ampacity limit. I think a vendor rep for a company that sells elbows would be a good source for an answer.

However, my question is why the contractor wants to use 2150 amp fuses. At 4160 volts, a 750 KVA transformer has a secondary current rating of about 104 amps. I know that table 450.3(A) allows you to go up to 250% of that, and in this case that would be around 250 amps. But why would they need to go that high? I would recommend calling for 200 amp fuses, under the presumption that the elbows need to be protected at that value or lower.
 
Disclaimer, I am not a MV guy. Maybe someone like @Hv&Lv will see this. My first thought is there is nothing wrong with it. First of all, there is no "everything must be protected at it's rating" clause in the NEC. Second, MV conductor protection is often higher than we are used to at LV.
 
charlie b said:
How abut 240.4?
Click to expand...
Which is titled "protection of conductors". Kind of have to assume this means art 310 conductors and not necessarily devices and other connected accessory items.

Unless otherwise instructed to comply whith this elsewhere.
 
IMO the cable terminations and joints should match the cable ampacity.
What size cable are you using?

these terminations are rated for 200 amps continuous load, and only (10) 200 amp load break operations.


I’m with Charlieb above. 100% load is 104 amps. Why fuse that high? Inrush shouldn’t be that big of a problem on a 750kva.
Just because you can go 250% doesn’t mean you have to..
 
charlie b said:
How abut 240.4?
Click to expand...
Charlie, note part IX of 240 applies to MV. Again, I am not a MV guy, but it appears the NEC allows OCPD settings for MV to be based on analysis such as conductor damage curves and not just ampacity of the conductor. Note that 240.101 states the setting of a fuse shall not exceed three times the ampacity of the conductor.

When an MV question comes up, I always have to force myself to throw out everything I think I know and am used to with LV 😉
 
I am a MV guy. There are actually 3 current ratings. The 200 A is available as load break and non-load break. 600 A is only non-load break. Plus a bunch of voltage classes. It is not dead break. As long as the transformer is unloaded removal is fine, same as a cutout. Plus unless you absolutely need load break I recommend against them. If you read the specs carefully they can require up to 150 lbs of force to remove where non-load break is half that. When the load break elbows get a little “seasoned” they take a two man crew or one man and a tow strap to remove.

A fuse rating may mean something when it protects a power cable but it is quite another thing when it is protecting an inductive load. They are never rated for 100% of the continuous duty rating due to inrush and temporary overloads. They will trip every time on startup. Effectively the downstream protection protects from overloads and the upstream protects against short circuit. So you really need to look at the continuous duty rating of the system not just the high side.

S&C (sandcastle.com) has a free program called Coordinaide that plots fuse curves for MV power fuses and transformer damage and magnetization curves for MV. It saves a lot of headaches if you don’t have access to power system analysis software and need to size fuses to avoid nuisance tripping.
 
paulengr said:
I am a MV guy. There are actually 3 current ratings. The 200 A is available as load break and non-load break. 600 A is only non-load break. Plus a bunch of voltage classes. It is not dead break. As long as the transformer is unloaded removal is fine, same as a cutout. Plus unless you absolutely need load break I recommend against them. If you read the specs carefully they can require up to 150 lbs of force to remove where non-load break is half that. When the load break elbows get a little “seasoned” they take a two man crew or one man and a tow strap to remove.

A fuse rating may mean something when it protects a power cable but it is quite another thing when it is protecting an inductive load. They are never rated for 100% of the continuous duty rating due to inrush and temporary overloads. They will trip every time on startup. Effectively the downstream protection protects from overloads and the upstream protects against short circuit. So you really need to look at the continuous duty rating of the system not just the high side.

S&C (sandcastle.com) has a free program called Coordinaide that plots fuse curves for MV power fuses and transformer damage and magnetization curves for MV. It saves a lot of headaches if you don’t have access to power system analysis software and need to size fuses to avoid nuisance tripping.
Click to expand...


I think you meant to type sandc.com, and it auto-corrected to sandcastle.
 
paulengr said:
I am a MV guy. There are actually 3 current ratings. The 200 A is available as load break and non-load break. 600 A is only non-load break. Plus a bunch of voltage classes. It is not dead break. As long as the transformer is unloaded removal is fine, same as a cutout. Plus unless you absolutely need load break I recommend against them. If you read the specs carefully they can require up to 150 lbs of force to remove where non-load break is half that. When the load break elbows get a little “seasoned” they take a two man crew or one man and a tow strap to remove.

A fuse rating may mean something when it protects a power cable but it is quite another thing when it is protecting an inductive load. They are never rated for 100% of the continuous duty rating due to inrush and temporary overloads. They will trip every time on startup. Effectively the downstream protection protects from overloads and the upstream protects against short circuit. So you really need to look at the continuous duty rating of the system not just the high side.

S&C (sandcastle.com) has a free program called Coordinaide that plots fuse curves for MV power fuses and transformer damage and magnetization curves for MV. It saves a lot of headaches if you don’t have access to power system analysis software and need to size fuses to avoid nuisance tripping.
Click to expand...

30+ years of pulling elbows we have never had an elbow that required a “towstrap”. They never get that sealed. That’s why they make slide hammers.
And with the newer design you don’t have to worry (as much) about partial vacuum flashover on the load break elbows.

The manufacturer calls 600 & 900 dead break. We will not take them off hot. I would really hate to drop that bolt out with a dead break elbow and have to put it back on with 14,000 volts (or even 4160) while standing on the ground. Only a fool would try that.

As far as fusing, a 100T will hold 400 amps for 5 seconds, 1000 amps for 42 cycles. Plenty of time for inrush to settle down.

I think a bigger problem may be getting the fuse selection on a transformer that size to coordinate with the utility protection.
 
Hv&Lv said:
30+ years of pulling elbows we have never had an elbow that required a “towstrap”. They never get that sealed. That’s why they make slide hammers.
And with the newer design you don’t have to worry (as much) about partial vacuum flashover on the load break elbows.

The manufacturer calls 600 & 900 dead break. We will not take them off hot. I would really hate to drop that bolt out with a dead break elbow and have to put it back on with 14,000 volts (or even 4160) while standing on the ground. Only a fool would try that.

As far as fusing, a 100T will hold 400 amps for 5 seconds, 1000 amps for 42 cycles. Plenty of time for inrush to settle down.

I think a bigger problem may be getting the fuse selection on a transformer that size to coordinate with the utility protection.
Click to expand...
Where does one find 600 amp connectors? Is it just a matter of doing the math? Looks like for 15 kv class you would need a monster 5000 kva to need one. At 4160 of course you get there much sooner, 1,400KVA.
 
They can be ordered easily enough.
200 amp loadbreak elbows, around $30-50 apiece. I think we pay about 25-28 because we use so many.
600 amp kits.. $75

Think feeders with several transformers rather than just one.
or entire circuits with about 100 transformers in the75-1500 kVa range.
 
Hv&Lv said:
They can be ordered easily enough.
200 amp loadbreak elbows, around $30-50 apiece. I think we pay about 25-28 because we use so many.
600 amp kits.. $75

Think feeders with several transformers rather than just one.
or entire circuits with about 100 transformers in the75-1500 kVa range.
Click to expand...

Yes, when there are several transformers, its common to loop through each xformer, so each elbow may carry the load of several transformers. Each transformer might have a loop switch with "A", "B", "A&B", and "loop through" positions.

And if we do this at 4160V, several transformers will exceed the 200A limit really quick.
 
msteiner said:
I'm reviewing a submittal for load break elbows on a 4160V system. The contractor is proposing 200A elbows, but the upstream protection is 250A fuses. Any concerns with using these elbows?
Click to expand...
I know that 600 Amp non-load break elbows do have a short time overload rating on some manufacturer's equipment. For some Eaton products the overload allowed on a 600 amp elbow is 1000 amps for 24 hours.

On most 200 amp load break elbows have no overload rating but do have short time withstand ratings such as like 10,000 amps for 0.17 seconds or 3,500 amps for 3 seconds. All based on thermal damage to the elbows insulation quality and IEEE 386.

NEC section 215.3 however states the general requirement is to size the OCP no less than 125% of the continuous load and 100% of the non-continuous load. So 125% of 200 Amps is 250 Amps if this helps.
 
Status
Not open for further replies.
Top