0.167ms vs. 1/2 cycle

[tulum]

Stupid question...

- 1 cycle is 1/60 = 16.7ms

- From previous experience I understood that the "Rated Short Circuit Withstand Current" = "Momentary Current" = "Making current" for a switch and = "Close and Latch" for a breaker....

Why is the Rated Short Circuit Withstand Current based on 0.167s (10 cycles) and the Momentary Current 1/2 cycle (0.084ms)?

What is missing?

Regards,
TULUM

 

[rbulsara]

They are not the same.

1/2 cycle momentary duty takes into account maximum 'asymmetry' (higest peak) for making. The most severe duty the contacts may have to perform, closing on a fault.

The withstand rating reflects 'symmetrical' value of SC, as the DC component decays in first 2-3 cycles, but the breaker itself does not open, and gives the downstream breakers/fuses a chance to open. The assembly have to withstand the mechanical forces generated by SC during this period but not open the contacts. The two duties are different.

 

[tulum]

rbulsara,

I am still not getting it...

The Rated Short Circuit Withstand Current as defined by IEEE/ANSI C37.100 is...

"RATED SHORT-CIRCUIT WITHSTAND CURRENT: The rated short-circuit withstand current of metal-enclosed bus is the maximum rms total current that it can carry momentarily without electrical, thermal, or mechanical damage or permanent deformation. The current shall be the rms value, including the direct-current (dc) component, at the major peak of the maximum cycle as determined from the envelope of the current wave during a given test time interval." (It's duration is 167ms, units are kA asym.)

To me this is pretty close to the momentary definition.

"MOMENTARY CURRENT: The current flowing in a device, an assembly, or a bus at the major peak of the maximum cycle as determined from the envelope of the current wave."

HELP!

Regards,
TULUM

 

[dpc]

I think the definition of "withstand" depends on the equipment you are talking about and the test standard being used.

Bus is rated differently than circuit breakers.

Some breakers and switches test standards specify that the device be able to withstand a certain level of current for a certain length of time without coming apart and without operating.

For low voltage power circuit breakers, I believe the duration is 30 cycles. For medium voltage metal-clad switchgear, a 3-second rating is given. These values are generally in symmetrical amps and are not equal to the close & latch rating.

UL-listed molded case circuit breakers have no withstand rating, only an interrupting rating.

 

[rbulsara]

I still maintain my opinion..

Regarding the definitions, I would say the dc component after 3 cycles gets very small..(depends where the fault is on the system and what voltage) The word momentarily in the first definition may be misleading as that 'moment' is several cycles..

In the definition of 'Momentary Current", 'major peak of the maximum cycle' effectively amounts to first 1/2 cycle.

 

[RalphChristie]

I agree with rbulsara and dpc.
In my opinion:

MOMENTARY CURRENT, max current peak in a circuit at any time. An exponential decaying current component (DC decrement) will be added to the AC symmetrical component forming the total asymmetrical current wave at a particular instant. Max peak is in the first half-cycle (zero to max) due to the dc-offset.

RATED SHORT CIRCUIT WITHSTAND CURRENT, max rms current a breaker can withstand without any damage, linked to a certain time. Like dpc said, different equipment have different time ratings. (Medium voltage circuit breakers are normally rated for 3sec.)

I am not in the IEEE/ANSI world, but in the IEC-world. Although they use different calculations, methods, standards and terms to describe certain currents, the physics stay the same. See thread238-81200 for some explanations of IEC short-circuit rating terms. Maybe it can be of some use.





Regards

Ralph