NEC 240.86

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tmmccauley

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Hi Mike:

I have a question about NEC 240.86.

My question is about the series rating of MCBs in series with current limiting fuses. UL has published a number of tables for acceptable series rating combinations of current limiting fuses and MCBs. NEC 240.86 says that the series rating of a breaker/breaker series combination can not be used if the motor contribution to the bus between the current the upstream protective device and the protected MCB is greater than 1% of the interrupting rating of the MCB. I don't understand the logic here.

If UL has tables of tested fuse/MCB acceptable series combination don't they still apply to the fuse/MCB combination. I understand those tables to say that the downstream MCB remains passive while the current limiting fuse interrupts the SC current. If the motor contribution, which I understand will continue to flow after the fuse has interrupted the main system contribution, is less than the interrupting rating of the breaker (which will interrupt the motor contribution) why can't I use the series ration of the fuse/MCB combination?

Thanks for your help.

Tom McCauley
 
I think you are missing a big part of 240.86(C) and that is the motor would be located on the load side of the upstream overcurrent device and the line side of the downstream series rated overcurrent device.

If the motor loads are all on the load side of the series rated overcurrent device then 240.86(C) does not apply and a series rating is permitted.

240.86 Series Ratings.
(C) Motor Contribution. Series ratings shall not be used
where
(1) Motors are connected on the load side of the higherrated
overcurrent device and on the line side of the
lower-rated overcurrent device, and
(2) The sum of the motor full-load currents exceeds 1 percent
of the interrupting rating of the lower-rated circuit
breaker.
Click to expand...

The inclusion of the word "and" at the end of 240.86(C)(1) makes it inclusive of (2) so both conditions are required before series rating is prohibited.

Hope this helps.

Chris
 
FWIW,
if
I understand the 2011 240.86 and that your fuse is followed by a motor is followed by an MCB, and
MCB IR < fuse IR
and
MCB I^2 T curve > fuse I^2 T curve
then
the 1% seems to be a safety factor such that the motor full load currents that are passing through the downstream MCB never approach the IR of the downstream MCB so there is virtually no possibility that the downstream MCB will try to interrupt the circuit.
This addt'l constraint is probably necessary because
MCB I^2 T curve > fuse I^2 T curve.
 
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G._S._Ohm said:
....
the 1% seems to be a safety factor such that the motor full load currents that are passing through the downstream MCB never approach the IR of the downstream MCB so there is virtually no possibility that the downstream MCB will try to interrupt the circuit.
This addt'l constraint is probably necessary because
MCB I^2 T curve > fuse I^2 T curve.
Click to expand...

In series combinations, there is no guarantee that the downstream device will not try to interrupt the circuit. In 'tested combinations' it makes absolutely no difference with protective device interrupts the fault.

However you are correct, the 1% is a safety factor dealing with motor fault contribution that does not flow through the upstream device. This factor is a comprise for determining the actual fault contribution of motors and their affect on the circuit.
 
Need Clarification

Need Clarification

Gentlemen:

After thinking about your comments I have some more questions and concerns.

I am completely aware of240.86(C). That was why I asked my original question.

Jim, you say that in a series combination of breakers or fuse and breaker it does not make any difference which over current device clears the fault. That means your saying it's OK if the under-rated downstream breaker (that is being protected by the upstream breaker/fuse) clears the fault before the upstream breaker does. If that's the case, why do we need the higher rated upstream breaker?

My understanding is that the upstream higher rated over current device (breaker/fuse) is used to protect the lower rated downstream breaker by clearing the fault before the under-rated downstream breaker tries to do so.

I'd welcome any further clarification anyone may have on why 240.86(C) should be used to prevent the use of series ratings.
 
My understanding is that the upstream higher rated over current device (breaker/fuse) is used to protect the lower rated downstream breaker by clearing the fault before the under-rated downstream breaker tries to do so.
Click to expand...

Not necessarily, the upstream overcurrent protective device is tested so that it will limit the amount of current flowing to the downstream breaker to less then the AIC rating of the protected OCPD. There is nothing in the testing that says the downstream device can't interrupt the fault at a lower current level.

Hope that makes sense.:)

Chris
 
Raider 1

Raider 1

That's exactly what I'm getting at. A current limiting fuse reduces the fault current that the protected breaker must interrupt. And if that lowered current combined with any motor contribution is less that the AIC of the protected breaker that should be an acceptable use of a current limiting fuse.
 
So we have four variables for two OCPDs and a motor load - the I?T rating, the Interrupting rating, the current limiting function and the motor current?
This is many combos and I guess some of them are forbidden. This gets messy.
 
tmmccauley said:
Jim, you say that in a series combination of breakers or fuse and breaker it does not make any difference which over current device clears the fault. That means your saying it's OK if the under-rated downstream breaker (that is being protected by the upstream breaker/fuse) clears the fault before the upstream breaker does. If that's the case, why do we need the higher rated upstream breaker?

My understanding is that the upstream higher rated over current device (breaker/fuse) is used to protect the lower rated downstream breaker by clearing the fault before the under-rated downstream breaker tries to do so.
Click to expand...

There is absolutely no doubt that a downstream device can begin to open prior to an upstream device clearing the fault. This is part of the effect called 'dynamic impedance', which is why even fuse manufacturers warns against using the "up-over-down" method with molded case breakers.

Another issue has to do with marketing strategies and cost of Listing testing. For example, UL might have a cutoff level of 10kAIC @208V, so there is no cost effective reason to test a standalone breaker at any higher fault value even though the breaker might be able to handle 18kA.

Most breaker manufactures publish selectivity tables showing series-combinations of breakers that can be used to simultaneously meet the requirements of 240.86 and 700.27.
 
tmmccauley said:
That's exactly what I'm getting at. A current limiting fuse reduces the fault current that the protected breaker must interrupt. And if that lowered current combined with any motor contribution is less that the AIC of the protected breaker that should be an acceptable use of a current limiting fuse.
Click to expand...

http://www.iaei.org/magazine/2004/11/new-requirements-for-series-rated-systems/
"It should be noted that, at this time, there is not a generally accepted method to compare the let-though characteristics of a current-limiting fuse or current-limiting circuit breaker with the interrupting rating of modern molded case circuit breakers that exhibit dynamic impedance during the first ? cycle after the initiation of a fault."
 
jim dungar said:
http://www.iaei.org/magazine/2004/11/new-requirements-for-series-rated-systems/
"It should be noted that, at this time, there is not a generally accepted method to compare the let-though characteristics of a current-limiting fuse or current-limiting circuit breaker with the interrupting rating of modern molded case circuit breakers that exhibit dynamic impedance during the first ? cycle after the initiation of a fault."
Click to expand...
Thanks for the link. Apparently, dynamic in this sense means changes with time. Here's another example, on page 17 of this link.
http://static.schneider-electric.us/docs/Electrical Distribution/0100DB0603.pdf
 
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