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What Do Symmetrical, Asymmetrical,

Momentary, Interrupting, Close &

Latch Ratings Mean? By  Admin On June 7, 2011 · 2 Comments

These terms were coined over the years to identify the appropriate short circuit current

magnitude that engineers could use to rate the switchgears, switchboards, panelboards,

circuit breakers, fuses, etc.

Without delving into details, the following is a simple explanation of these terms andwhen they are used. Note that these ratings are current ratings.

Symmetrical (RMS) Current

This term is widely used to identify the short circuit rating of breakers in low voltage

systems. This is because the low voltage breakers take 8 to 10 cycles to break a circuit. In

8 or more cycles (typically 15), the fault current will decay to a symmetrical waveformwhich, ofcourse, would have no DC offset.

Low voltage panels too are rated by their symmetrical current rating.

Most modern circuit breakers implicitly list their ratings in symmetrical amps.

Asymmetrical (RMS) Current

During the first half of a cycle, the fault current is at its largest magnitude – occurring at a

moment when the voltage wave is passing the reference axis. This asymmetry is brought

on by the DC offset current (see Figure 1 below.) At the half cycle mark, the peak RMS

value of the asymmetrical current is about ~1.6 times the symmetrical current.

Keep in mind that this magnitude is NOT the same as peak asymmetrical current. In

1987, the IEEE committee established this term (peak asymmetrical) to make it clear to

the manufacturers that it is the peak current magnitude and not the RMS peak that

generates destructive forces. How destructive? Well, peak asymmetrical current is about

~2.7 times the symmetrical current.

Realistically, the RMS quantity of any AC signal (voltage or current) is a phantom

quantity. It is created to compare the AC magnitude to that of the DC i.e. making sureyou are comparing apples to apples.

So, for example, a 5Amp RMS AC current is the same as a 5Amp DC current. However,

the peak value of the AC quantity will be .

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Figure 1: Fault current magnitude following a fault

Momentary Rating

This rating is used in medium voltage and high voltage systems especially when callingout switchgears, switchboards and circuit breakers.

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This is a half cycle rating and quite similar in meaning to peak RMS asymmetrical amps

except it does not have the same unwieldy name.

Interrupting Rating

Quintessentially associated with medium voltage and high voltage circuit breakers andfuses.

It is the short circuit current a protective device can safely interrupt typically in 2 cycles

to 5 cycles. The fault current magnitude is less than the momentary currents since the

fault current would have decayed in this duration.

Interrupting rating can be specified as short circuit MVA too. This is given by thefollowing:

where,

= Line to line voltage in kV

= Interrupting current in kA

Close & Latch Rating or Making Current

Again, used for high voltage and medium voltage circuit breakers. It is the capability of 

the breaker to close into a fault and stay in that position without destroying its poles. Thisrating is same as the peak asymmetrical current (during the first cycle) after the breaker 

closes.

Typically, in HV systems, the circuit breakers are programmed to automatically reclose

after opening for approximately 15 to 50 cycles (depending on the operating voltage.) If the fault condition disappears in this duration then the breaker will remain closed

otherwise the breaker is tripped and blocked from reclosing. Essentially, this rating

determines if the breaker can stay closed or not.

One cycle is ‘ th of a second, assuming the system frequency is 60hz