Choosing the between Inverse-time and Instantaneous-trip Breaker.

[Frank A]

I've got to install a main breaker on a main panel for a 117A dwelling unit. I've doubts on which breaker should I install: an Inversed-time one or an Instantaneous-trip one.

At first, I decided to install a 125A inversed-time breaker by NEC 240.6, but I got confused if an instantaneous-trip should be use in this case. This is mainly because I yet don't know the characteristics that should be met in order to use any.

Which one do you think I should install?

 

[Ingenieur]

Inverse time is the std for most applications
combines time over current (thermal or digital equivilent) and instantaneous/magnetic/short ckt

instantaneous (mag only no thermal) only will not provide time over current for conductor protection

instant. typical on motors with seperate overloads

 

[Frank A]

Inverse time is the std for most applications
combines time over current (thermal or digital equivilent) and instantaneous/magnetic/short ckt

instantaneous (mag only no thermal) only will not provide time over current for conductor protection

instant. typical on motors with seperate overloads

Thanks, Ingenieur. That was the answer I was looking for.

 

[Jraef]

You cannot legally use an instantaneous-only circuit breaker anyway. The ONLY acceptable use for them is as a component in a FACTORY built and NRTL listed combination starter assembly. You cannot even fabricate your own starter in the field with them. There is absolutely no valid way to use one as a main breaker at all. Thermal-mag or electronic trip are your only choices.

I just noticed you are in Venezuela, the code requirements I mentioned are relevant to North America, so you would need to determine how that works in your country.

 

[Ingenieur]

Instant. breakers are standard in mine power load centers
used ug and on the surface
they are used as feeder protection not in a combo motor starter
they are not built to ul or any other nrlt standard
they are legal

 

[big john]

The NEC doesn't have providence over mines. Check out 90.2(B).

That said, if all your breakers are magnetic trip only, how do you guard against overloads?

 

[Ingenieur]

The NEC doesn't have providence over mines. Check out 90.2(B).

That said, if all your breakers are magnetic trip only, how do you guard against overloads?

I know this 'underground' only though (I work for a gov mining enforcement bureau)
we use it at our discretion

but it's a grey area for surface facilities
the last nec code adopted by MSHA was 1968 and still in effect
coal, mineral non-coal, ie limestone, etc

extrrnal overload packs

 

[Haji]

Instant. breakers

If they are fixed type, motor protection may be compromised.

 

[Ingenieur]

If they are fixed type, motor protection may be compromised.

obviously it must be sized correctly if fixed
most are adjustable

it does not protect the motor
the ol's do that
although I guess you could say the cb may trip first if locked rotor i is sustained

but even an it cb will take some time
assume motor fla of 50
and a 100 A cb
at 600% motor fla (300% cb rating) it might take 10-30 sec
what's a class 2 overload take? Edit at 600% about the same 20 sec

 

[Haji]

obviously it must be sized correctly if fixed
most are adjustable

it does not protect the motor
the ol's do that
although I guess you could say the cb may trip first if locked rotor i is sustained

but even a it cb will take some time
assume motor fla of 50
and a 100 A cb
at 600% motor fla (300% cb rating) it might 10-30 sec
what's a class 2 overload take?

The instantaneous setting may be higher than short circuit current in motor and so the instant breaker won't operate but O/L relay may. But O/L relay may not have sufficient breaking capacity for short circuit current. So both motor and O/L relay would likely be damaged.

 

[Ingenieur]

The instantaneous setting may be higher than short circuit current in motor and so the instant breaker won't operate but O/L relay may. But O/L relay may not have sufficient breaking capacity for short circuit current. So both motor and O/L relay would likely be damaged.

it will likely be higher if not selected properly
then again it is usually sized per the nec table
which does not account for line Z fault reduction
so oversized
and allowed to be set at 1100% or 1300 under some cases
the ol will always trip first in the case

in fact most it cb's instant. region is the same, 1200-1300%
again
50 fla
100 it cb
instant trip >1100% or 1100 A
locked rotor 300 A perhaps
ol's catch it either way

the overload does not break load
a properly rated contactor is rated to break locked rotor current

not an issue if the code is followed

 

[Haji]

it will likely be higher if not selected properly
then again it is usually sized per the nec table
which does not account for line Z fault reduction
so oversized
and allowed to be set at 1100% or 1300 under some cases
the ol will always trip first in the case

in fact most it cb's instant. region is the same, 1200-1300%
again
50 fla
100 it cb
instant trip >1100% or 1100 A
locked rotor 300 A perhaps
ol's catch it either way

the overload does not break load
a properly rated contactor is rated to break locked rotor current

not an issue if the code is followed

1) I can't believe O/L relay even through a contactor can clear motor short circuit current as both have limited breaking capacity.
2)Motor failure can result due to wrong breaker setting even when code is followed.

 

[Ingenieur]

The instantaneous setting may be higher than short circuit current in motor and so the instant breaker won't operate but O/L relay may. But O/L relay may not have sufficient breaking capacity for short circuit current. So both motor and O/L relay would likely be damaged.

The ol and starter are rated to break the current
the ol is rated to trip to PREVENT damage
in most cases it will trip before the inv time

consider
460/3 25 hp
fla table = name plate = 34 A
use inst at 800% or 275 A assume adjustable
ol 42 A 125%
code E lra 157 A
cb will never trip, but is code compliant
meaning the ol is counted on for protection

motor lr Z = 460/157 = 2.9 Ohm
at 3% v drop conductor Z = 0.6 Ohm
total Z = 3.5 Ohm
Actual sc or lr fault with motor in ckt 460/3.5 = 131 A
still won't trip

% ol setting 131/42 = 310%
class 20 trip time 30 sec (rated to 10 times setting ~10 sec)

it cb
250% or 85 A use 90
trip time at 131 A or 150% frame
typ 70-300 sec
ol will trip first anyways

it is a non-issue as long as code compliant

 

[Ingenieur]

If a sc occurs in the feeder (assuming at motor or winding dead bolted)
460/0.6 = 760 A

instant cb is 275 so trips without delay

it
760/90 = 850%
not in the inst range of most std it cb's
might take 1-3 sec vs 3-5 60 hz cycles for the instant cb
but definitely no faster than the instant

 

[Haji]

The problem is if the instant element also in inverse breaker is fixed type (permitted by code), it may compromise motor protection.

 

[iwire]

The instantaneous setting may be higher than short circuit current in motor and so the instant breaker won't operate but O/L relay may. But O/L relay may not have sufficient breaking capacity for short circuit current. So both motor and O/L relay would likely be damaged.

If there is a short in the motor it is already damaged and there is nothing left to protect.

 

[Sahib]

If there is a short in the motor it is already damaged and there is nothing left to protect.

To prevent further damage.
You have no idea.

 

[iwire]

You have no idea.

When you disagree with me I know I am on the right track.

 

[Sahib]

When you disagree with me I know I am on the right track.

You are misleading yourself.

 

[wbdvt]

When you disagree with me I know I am on the right track.

+1 agree with you