FAULT
CURRENTS
A
common type of electrical failure is for an electrically
energized part or conductor to come into contact with
a grounded case or shield. In the case of a cable, this
would take place if there were an insulation failure.
When
this type of fault develops, large electrical currents
may be expected to flow (normally called fault or short
circuit currents) from the energy source (usually a
substation), through the wires or cable conductors,
through the fault, and through all available ground
paths back to the substation.
Of
course, substations have protection against this sort
of thing and when this large current is sensed by relaying
devices, circuit breakers open to disconnect the faulted
circuit. If there are fuses between the substation and
the fault, these will remove the circuit.
The size of the fault currents will depend on:
- The
electrical power capacity of the substation.
- The
size and types of wires and cable between the substation
and the fault.
- Distance
between the substation and fault.
The time interval that these short circuit currents
will flow depends on the time required for breakers
or fuses to operate.
A grounded cable shield is one of the ground paths
used by the fault current. Neither the conductor nor
shield is sized to carry currents of this magnitude
for any length of time. The instant a fault current
begins to flow, the conductor and shield begin to over-heat.
If
the fault current is not interrupted in time an interesting
series of events and consequences can occur.
- The
insulation and jacket can be badly damaged.
- Portions
of the conductor and shield in the vicinity of the
fault can be burned away.
- It
is possible that burning is so rapid that the fault
path is burned away which in turn quickly reduces
the fault current. This situation can lead to interesting
but serious consequences:
- The
relays or fuses might not sense the fault.
- The
resulting lower but still excessive current might
damage other parts of the system.
- This
will occur until a fault current large enough to
actuate the relays is obtained.
These
factors either separately or in combination are the
major basis for concern by cable users.
Note
that it is not only the amount but also the time that the fault current exists that is important
The
ability of a conductor or shield to carry fault current
increases with AWG size (amount of metal). Unfortunately
this is just opposite of what is desirable from a shield
loss standpoint
Logically
it would appear that a study in depth or comparison
of shielding systems is in order, This could be very
helpful if every possible fault condition were plugged
into this study.
A
more practical solution for proper cable selection can
be based on consideration of the following.
- Shield
losses are an every day and continuing expense.
- Faults,
hopefully, are an occasional event.
As
a consequence of these:
- Economics
would dictate selection of shield size which would
be adequate to provide sufficient fault current
to operate relays - but no more than this to minimize
shield losses.
- It
appears that a moderate amount of cable damage (in
vicinity of the fault) could be tolerated as long
as the damages did not "spill over" into
the rest of the system.
There
is no hard and fast set of technically accepted criteria
that dictates that all of the metal required for adequate
fault capability has to be in the shield.
If
this approach is agreeable there are means through new
cable concepts to insure adequate fault current capability
consistent with minimum shield losses. Cables are being
evaluated under actual fault conditions to substantiate
this concept.
