Cold Sequence Meter Disconnects for Fire Pumps

[JoeStillman]

More and more utilities are requiring cold-sequence metering, and they don't want an overcurrent device ahead of the meter. But non-fused disconnects are only good for 10 kA SCCR. It's a rare fire pump service indeed that is that soft.

1. Is there a class of fuses that are for Short-Circuit only, and not Overload?
2. Has anybody used cable limiters in this situation?
3. Would these fuses raise the AIC rating of a fusible disconnect?

 

[synchro]

They are somewhat pricey, but Larson Electronics offers service rated non-fused disconnects, some with a 100 kA rating and a few with 200 kA.

https://www.larsonelectronics.com/search/result?w="MPD-DS" "Non-Fused Manual AC Disconnect"

 

[MyCleveland]

More and more utilities are requiring cold-sequence metering, and they don't want an overcurrent device ahead of the meter. But non-fused disconnects are only good for 10 kA SCCR. It's a rare fire pump service indeed that is that soft.

1. Is there a class of fuses that are for Short-Circuit only, and not Overload?
2. Has anybody used cable limiters in this situation?
3. Would these fuses raise the AIC rating of a fusible disconnect?

My solution to deal with this was a Molded Case Switch...SE Labeled. No overload trip on a utility locked switch, but has Instan Trip. I am sure you know....just select the type with the AIC you need.

 

[electrofelon]

More and more utilities are requiring cold-sequence metering, and they don't want an overcurrent device ahead of the meter. But non-fused disconnects are only good for 10 kA SCCR. It's a rare fire pump service indeed that is that soft.

1. Is there a class of fuses that are for Short-Circuit only, and not Overload?
2. Has anybody used cable limiters in this situation?
3. Would these fuses raise the AIC rating of a fusible disconnect?

What is the issue with just using a regular fused switch?

 

[JoeStillman]

What is the issue with just using a regular fused switch?

For fire pumps, the fuses have to be able to carry locked rotor current indefinitely. It makes the switch 6x bigger than I would like. Also, one of the utilities doesn't want any overcurrent protection in a meter disconnect.

 

[Hv&Lv]

May be a stupid question, but wouldn’t the utility fuse ahead of the utility bank blow first on indefinite locked rotor current?
Or is this customer supplied equipment?
Either way, the utility is fused or relay protected also

 

[JoeStillman]

May be a stupid question, but wouldn’t the utility fuse ahead of the utility bank blow first on indefinite locked rotor current?
Or is this customer supplied equipment?
Either way, the utility is fused or relay protected also

There is almost never any fuse between the utility transformer secondary and the meter.

 

[Hv&Lv]

There is almost never any fuse between the utility transformer secondary and the meter.

No, I meant ahead of the bank on the primary side. Also bayonets in padmount.
We fuse for bank, not load

I don’t recall an EC ever asking me about our primary fusing until it blows.

 

[Jpflex]

May be a stupid question, but wouldn’t the utility fuse ahead of the utility bank blow first on indefinite locked rotor current?
Or is this customer supplied equipment?
Either way, the utility is fused or relay protected also

Your not talking about utility power pole fuse cutouts feeding utility transformer primary are you?

Is it normally the lineman (assuming older style overhead power poles) cutout fuses that serve as primary OCPD for utility transformer?

 

[Hv&Lv]

Your not talking about utility power pole fuse cutouts feeding utility transformer primary are you?

Is it normally the lineman (assuming older style overhead power poles) cutout fuses that serve as primary OCPD for utility transformer?

Yes. Exactly.
We have 20 amp primary fuses on banks that have a couple of 600A disconnects on the secondary side.

We
Had a mill that added two 600HP motors and didn’t inform us about the change.
They were tripping out the recloser on the line at the station.
Only after we realized what was going on and discussed some things with them did we raise the pickup settings

 

[JoeStillman]

Actually, the primary fusing makes a difference in my arc-flash calcs. If I assume there is no fusing, I get a red "unsafe for live work" sticker at the CT cabinet, but with it, almost always an orange one (under 40 cal).

 

[Jpflex]

Yes. Exactly.
We have 20 amp primary fuses on banks that have a couple of 600A disconnects on the secondary side.

We
Had a mill that added two 600HP motors and didn’t inform us about the change.
They were tripping out the recloser on the line at the station.
Only after we realized what was going on and discussed some things with them did we raise the pickup settings

The first time I helped install load breaks to supply primary to a 3 phase 300 kva delta delta ungrounded transformer 4800/ 480 volts, the lineman apparently did not know what to do with the small exterior wires that came out of the load break boots.

I remember he tied all boots together in this way and bonded to the transformer cabinet ground. (System windings were ungrounded) I wish I had a picture to explain and could be shown the proper install for these?

 

[electrofelon]

For fire pumps, the fuses have to be able to carry locked rotor current indefinitely. It makes the switch 6x bigger than I would like. Also, one of the utilities doesn't want any overcurrent protection in a meter disconnect.

But don't the conductors have to be 6X too? When you need the bigger switch just to land the conductors?

 

[synchro]

For fire pumps, the fuses have to be able to carry locked rotor current indefinitely. It makes the switch 6x bigger than I would like. ...

Larson makes service rated non-fusible disconnects with a rotary handle that appear to be significantly smaller than the disconnects I posted earlier, although they don't provide any dimensions on their on-line spec sheet. They are pricey, though.
An example is:

https://www.larsonelectronics.com/product/287962/400-amp-non-fused-manual-ac-disconnect-safety-switch-250v-ac-3-pole-nema-1?productIdBase64=Mjg3OTYy0&rank=30

 

[JoeStillman]

Larson makes service rated non-fusible disconnects with a rotary handle that appear to be significantly smaller than the disconnects I posted earlier, although they don't provide any dimensions on their on-line spec sheet. They are pricey, though.
An example is:

https://www.larsonelectronics.com/product/287962/400-amp-non-fused-manual-ac-disconnect-safety-switch-250v-ac-3-pole-nema-1?productIdBase64=Mjg3OTYy0&rank=30

I saw that one. There is a disclaimer on the picture. I don't think the marketing department pulled the right photo out of the bin when they made that web page.

 

[JoeStillman]

But don't the conductors have to be 6X too? When you need the bigger switch just to land the conductors?

The conductors are not required to carry locked rotor current. See 695.6. If you have a 2017 handbook there's an interesting example on pages 948-949.

 

[jim dungar]

Actually, the primary fusing makes a difference in my arc-flash calcs. If I assume there is no fusing, I get a red "unsafe for live work" sticker at the CT cabinet, but with it, almost always an orange one (under 40 cal).

So 40.01cal/cm² is unsafe but 39.99 is perfectly safe?
These must be customer specific break points, because NFPA 70E does not contain any color coding.

I have never found utility side primary fuses to make much different on my customer's service equipment arc flash values.

 

[jim dungar]

May be a stupid question, but wouldn’t the utility fuse ahead of the utility bank blow first on indefinite locked rotor current?
Or is this customer supplied equipment?
Either way, the utility is fused or relay protected also

I don't think I have seen many utility fuses sized for protecting the transformer from overloads, for secondary service customers. Most of the ones in this area are to protect the line from a failed transformer.

 

[Hv&Lv]

I don't think I have seen many utility fuses sized for protecting the transformer from overloads, for secondary service customers. Most of the ones in this area are to protect the line from a failed transformer.

As a general rule we fuse ours to 200% of the transformer.
We don’t do it to protect the customer.
We do it to protect the transformer. Over size a fuse to 300-600% and it tends to make the tops blow off them when something happens.

 

[electrofelon]

The conductors are not required to carry locked rotor current. See 695.6. If you have a 2017 handbook there's an interesting example on pages 948-949.

Ok, I see. I remembered the locked rotor part for fire pumps but couldn't remember what components exactly had to be sized to that.