Page 436 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.36 THE WORK

FIGURE 9.30 Parallel hookup.

Caps made by different manufacturers must not be used in a series, because of variation in current
requirement for detonation.

Parallel. In a parallel circuit, Fig. 9.30, the current does not go through the caps one after another
but goes through all of them at the same time. A poor connection on a cap wire affects only that
cap. The voltage requirement is lower than when the same number of caps is shot in series, but
more amperage is needed.
Two caps in series have twice the resistance of one cap. Two in parallel have only one-half the
resistance of one, as less potential is required to force the current through a large conductor than
a small one. But where 1.5 amperes was sufficient current to shoot a whole series of caps, .5
ampere is required for each cap placed in parallel.
Parallel wiring is therefore preferred where a source with low voltage and high amperage, such
as a storage battery, is to be used. It is not suitable for blasting machines, and the results with dry
cell batteries are doubtful. High lines are equally efficient with either arrangement.
The most common simple parallel hookup is the second one shown, and it is not recognized as
such by many who use it. It is the most convenient way to fire a small irregular group of blasts.
In figuring a parallel circuit, the resistance on one cap is divided by the total number of caps,
and in a large blast, may be so small a figure that it can be ignored. The resistance of the two bus
wires, between the leads and the last cap, is approximately one-half the resistance of the same
length of the same wire used for a lead. Lead wire resistance is the same as in a series circuit.
The lowered resistance of the bus wires is due to the fact that some of the current is diverted
at each cap. Full current is present at the beginning, and zero current at the end, so that it averages
out to about one-half current for the full length of these wires.
Parallel Series. This layout, Fig. 9.31, makes it possible to shoot large numbers of caps with-
out requiring excessive voltage or amperage.
There is some disagreement about the balancing of the size of the different series. Technically,
each series should have the same number of caps. Many blasters, however, claim better results
when the series differ from each other by a set amount. This is said to be particularly advanta-
geous when firing an excessive number of caps with a blasting machine.
When the series are equal, and juice is put in the line, all caps are equally heated and should det-
onate simultaneously. If any series gets current but less than its share, due to a poor connection or
other defects, it may not fire. However, as the other series fire, current will cease to go through
them, and all of it will go through the remaining one, unless the wires are broken, until it fires also.
If the series have different numbers of caps, at first the current will flow most strongly through
the series with the fewest caps and least resistance, and having fired that, will concentrate on the