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General
questions / Short circuit currents
What is a short
circuit?
What is the prospective
short circuit current?
The prospective short circuit current is the value of the
current that would flow if there was no protection in the
circuit. The lower the power factor of the installation, the
higher the peak value of this destructive current.
Why is breaking
capacity important?
A protective device must be able to withstand the destructive
energy of short circuit currents. The rating which defines
the capability of a protective device to maintain its integrity
when reacting to fault currents is termed its breaking capacity
or "interrupting rating".
What is I2t
(Joule integral)?
Is it useful to have a low I2t?
Yes — the energy released during a short circuit, if
not limited, can strongly damage part or whole installations.
Two specific parameters affect the Operating I2t:
- Power factor
The lower the power factor the higher the energy, I2t.
- The voltage
The higher the voltage the higher the energy, I2t.
Fuselinks considerably limit this energy. For instance, without
a fuselink an asymmetrical short circuit current of 10,000
A at 230V a.c.could pass through the circuit during several
cycles. During the first cycle, the I2t could be as high as
4,000,000A2S.
In these conditions a 100A gG fuselink will limit the I2t
value to about 80,000A2s, i.e. only 2% of the value during
the first half cycle.
What is cut-off
current?
Current limitation depends on the short-circuit conditions
(prospective value, power factor, making angle at the beginning
of the short-circuit). Cut-off current characteristics show
maximum values of limited current reached in the worse conditions.
For a prospective short-circuit current of 10,000 Amp r.m.s.
with a totally asymmetrical current, the maximum value could
reach 25,000 Amp peak. A 100A gG fuselink limits the first
peak to 8,000 Amp, less than the third part of the prospective
maximum value. The destructive electrodynamic effects are
reduced by 90% (8,000/25,000) 2!
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