Page 195 - Handbook of Electrical Engineering

178 HANDBOOK OF ELECTRICAL ENGINEERING

time range of 0.01 to 1000 seconds. For times less than 0.01 sec it is better to seek the advice of a
particular manufacturer.
During the current limiting phase the operating time is influenced by whether the prospective
current is asymmetrical or fully symmetrical. The time is determined by the integrated amount of
heat generated and this is a function of the current waveform shape. To help overcome difficulties
in relating the terminology used in the non-current limiting phase to that applicable in the current
limiting phase, the term ‘virtual time’ was introduced some years ago.
Note that during current limiting operation the melting time and the arcing time are of the
same order, see Figure 8.2. The term virtual time (t v ) can be used in conjunction with the melting,
arcing and clearing times by using the following mathematical expressions:-

2
Melting energy (amp secs)
Melting t v =
Prospective current (rms sym amps) 2
1 t m 2
= i dt seconds
I 2 o

Similarly,

Arcing energy
Arching t v = 2
Prospective current
1 t c 2
= i dt seconds
I 2
t m
And,
1 t c 2
Clearing t v = i dt seconds
I 2 o

Where, t m is the melting time period.

t c is the clearing time period.
t c − t m is the arcing time period.

The manufacturers use this procedure to extrapolate their curves below 0.01 sec and t v is
therefore a theoretical time. Virtual time is related to the prospective current by definition and so
the manufacturer will quote the maximum prospective current that can be used in conjunction with
his curves. At this point the engineer is encouraged to consult the manufacturers for advice on the
selection of fuses for current-limiting duty.
The above discussion on current limiting and virtual time have been included for completeness
so that the reader is made aware of their significance.