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3. Technical and General Question Corner
Q. When considering earth
fault impedance, how are the values for the
maximum length of a circuit supplying socket
outlets determined when the circuit is protected
by a combined RCD and MCB (RCBO) that has an
earth fault threshold of 30 mA? The MCB is a
type C.
A. The central issue surrounding
earth fault impedance is the need to disconnect
the circuit within the safe time for the touch
voltages likely to occur.
It has been determined that where an earth
fault could give rise to a voltage in excess
of 50 volts AC the circuit must be disconnected
within 400 milli seconds for socket outlets.
To ensure the circuit breaker operates within
the 400 milli seconds its time current characteristics
must be analysed, however on average the instantaneous
portion of a Type C MCB will be in the order
of 7.5 times rated current.
Using the current and an operating voltage
of 230 volts the impedance required to produce
that can be calculated.
The calculated impedance represents the maximum
allowable circuit impedance, which then can
be translated into a maximum length for a circuit.
When an RCD is fitted to a circuit the operating
time of the RCD is independent of the time current
characteristic of the MCB and also invariably
much shorter. Therefore in such circumstances
there is no need to calculate the earth fault
impedance of such a circuit or determine a maximum
circuit length.
However the earth loop impedance value may
be useful for determining the earth resistance
required to ensure that the earthing system
is correctly connected and the circuit satisfies
the voltage drop requirements relative to its
length.
Q. What is touch voltage?
A. Touch voltage is the voltage
between the frame of equipment and other parts
a person can touch. Clauses 1.4.87, 1.4.4.1
and 1.7.4.3 of AS/NZS 3000:2000 refer to touch
voltages.
Q. I am an electrician and
if I ask for and receive advice from an electrical
inspector on how I should carry out prescribed
electrical work on an installation is that inspector
responsible for the work I do when the work
is done in accordance with the inspector's instructions?
A. The seeking of advice
from another person on how you should do your
work does not absolve you from your responsibilities
as an electrician.
If an inspector gives you advice which subsequently
makes your work non-compliant, that may be grounds
for mitigation but in general you are accountable
for all work you carry out You are also accountable
for the work a trainee carries out while the
trainee is assisting you.
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