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Cold Sequence Meter Disconnects for Fire Pumps

JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
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?
 
M

MyCleveland

Senior Member
Location
Cleveland, Ohio
JoeStillman said:
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?
Click to expand...
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

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
JoeStillman said:
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?
Click to expand...
What is the issue with just using a regular fused switch?
 
H

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
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

JoeStillman

Senior Member
Location
West Chester, PA
Hv&Lv said:
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
Click to expand...
There is almost never any fuse between the utility transformer secondary and the meter.
 
H

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
JoeStillman said:
There is almost never any fuse between the utility transformer secondary and the meter.
Click to expand...
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

Jpflex

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
Hv&Lv said:
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
Click to expand...
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?
 
H

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Jpflex said:
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?
Click to expand...
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

JoeStillman

Senior Member
Location
West Chester, PA
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

Jpflex

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
Hv&Lv said:
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
Click to expand...
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?
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
JoeStillman said:
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. ...
Click to expand...

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:

 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
synchro said:
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:

Click to expand...
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.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
JoeStillman said:
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).
Click to expand...
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.
 
Last edited:
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Hv&Lv said:
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
Click to expand...
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.
 
H

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
jim dungar said:
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.
Click to expand...
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.
 
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