39 / 52
DRIVES, MOTORS + SWITCHGEAR
ELECTRICAL PROTECTION + SAF TY
(or derating); Switching functions (switch, switch-disconnector,
Make-first-break-last); Mounting method and special environmental
requirements.
Current rating
For an MCB to be effective it must not trip (must hold) during normal
operation (load dependent) and must trip in time to protect cabling
during abnormal operation (cable dependent). These two conditions
can be expressed as I
n(MCB)
≥
I
n(Load)
and I
(CB Max Trip)
< I
n(Cable)
.
A common misconception is that a 100 A circuit breaker will not
trip at 99,99 A and will trip at 100,01 A. Any over-current device will
have actual trip-points different from its nominal rating. An MCB
certified to SANS-60947 [2] may have a non-tripping current (I
U
) of
105% its rating (I
n
) and a tripping current of 135%.
Start with the load to satisfy I
n(MCB)
≥
I
n(Load)
. If the circuit breaker is
to supply a known singular load, the load’s current rating can be taken
directly. In this case, the circuit breaker could be chosen to provide
protection against overload of the load equipment too.
If the circuit breaker is to supply a combination of loads, the circuit
breaker should be sized to supply the anticipated cumulative load.
This can become complicated for many intermittent loads or unknown
loads e.g. general plug sockets outlets. Sometimes the allowed load
is limited to a reasonable value, simply for economic reasons. In
this case, it is improbable that the circuit breaker will provide effec-
tive protection of the individual loads and therefore each load must
incorporate its own protection.
Also consider inrush effects. Many loads have an increased inrush
current that can last from milliseconds to minutes. A direct-on-line
motor can draw a start-up current of 5 to 10 times its rated current
when first energised. Instead of overrating the circuit breaker (and
subsequently the cable) by the same amount, utilise a circuit breaker
with a time-delay curve that exceeds the inrushmagnitude and period
of the load. This might require selecting a cable with a short time
overload rating in excess of the inrush (e.g. some PVC cables can
temporarily exceed the usual 70°C, up to 90°C or 105°C), otherwise
the useful life of the insulation will be impacted.
With the circuit breaker curve at hand and deratings (if any) ap-
plied, select a cable with appropriate cross sectional area, given the
insulation and installation method, to carry the worst case trip current
and satisfy I
CB Max Trip
< I
nCable
. For building installations, SANS-10142-1
[3] provides guidelines for calculating the conductor capacity. This
includes installation method, ambient temperature, number of cables
in proximity and insulation.
Using a slightly larger cable than neededmakes for a conservative
design. For very long cable runs, it might also be required to increase
the cable size to limit the voltage drop under full load. If the protected
circuit contains branches, each of these cables must individually cor-
relate to the circuit breaker rating, regardless of whether their normal
load will only be a fraction of the total.
Abbreviations/Acronyms
Voltage rating
It goes without saying that the
circuit breaker voltage rating
(U
e
) is determined by the source
voltage. Be sure to select a
breaker to the maximum work-
ing voltage of the system. It is
often overlooked that a lead-
acid battery with nominal 12 V
level can be charged over 14 V,
or that the open-circuit voltage
of PV solar panel array may
be significantly higher than its
nominal usage.
Correctly specify an ac or dc breaker
– these devices are generally not inter-
changeable. With dc the direction of cur-
rent flow is of importance too. Keep in mind
that in battery applications you would need a reverse feedable circuit
breaker, as the battery can both charge and discharge.
The over-voltage category is selected according to the position
in the installation. Surge energy is highest near the service entrance
and will dissipate towards lower levels of distribution. For usual
240 V distribution, preferred withstand voltage (U
IMP
) is 6 kV at the
service entrance, 4 kV in distribution and 2,5 kV at the load, but will
also rely on coordination with suitable Surge Protection Devices
(SPDs) throughout.
Short circuit rating
The source also determines the short-circuit rating (I
CU
and I
CS
) re-
quired, as the prospective short-circuit current depends on the source
voltage and source impedance. In a low voltage mains distribution
board, short-circuit currents of several kiloamperes is possible. Instal-
take note
• A circuit breaker is a critical safety device to protect
people and property where electricity is distributed.
• Its primary function is to protect the distribution system
from reaching damaging temperatures.
• These over-current devices protect us, our homes and
industries.
MCB – Miniature Circuit Breaker
PV
– Photovoltaic
PVC
– Polyvinyl Chloride
SPD
– Surge Protection Device
37
June ‘17
Electricity+Control