Products
-> Ancilliaries
-> Circuit Breakers and Residual Current Breakers
Carroll & Meynell Transformers Ltd can fit a wide
range of current tripping protection devices to our
enclosed transformer units. For enclosures rated up
to IP33 these units are mounted through the wall of
the enclosure. For higher IP rated enclosures the breaker
switches are situated behind weatherproof hinged covers.
Miniature Circuit Breaker (MCB) and Moulded Case
Circuit Breaker (MCCB)
(MCB’s) and Moulded Case Circuit Breakers (MCCB’s)
are resettable devices designed to protect the supply
or the transformer “Up-stream” of the device
from the effects of fault currents that occur “Down-stream”
of the device. These devices usually have two methods
of detecting and responding to an excessive current
that passes through them, “Thermal” and
“Magnetic”.
The Magnetic tripping part of the device responds to
the magnitude of the current passing through the device.
If the current exceeds a certain level the device will
trip.
The setting of this parameter will be for fault currents
associated with short circuits or excessive loads.
The Thermal tripping part of the device responds to
the I²t characteristics of the current i.e. a combination
of the magnitude of the current and the time for which
it is applied.
In this way large currents, greater than the device
rating but less than that required operate the magnetic
trip, lasting for short periods of time e.g. switch
on surges, will not trip a device while an overload
applied for an extended duration will trip the device.
The MCB devices are specified with different tripping
characteristics for their ability to withstand switch
on surges Type A, B, C or D curve, while the larger
MCCB devices often have this feature as an adjustable
characteristic that can be set during installation.
Residual Current Devices (RCD)
The Residual Current Device is a protection device
specifically designed to protect against the effects
of electric shock. MCB, MCCB and Fuses protect equipment
against the effects of overload either by the connection
of an excessive load to the system or a failure causing
an excessive fault current. The RCD operates by the
detection of a small (residual) current being lost from
the system through a high resistance, such as a person,
to ground.
RCD’s are rated for current and residual current.
The current they are rated for expressed in Amps represents
the maximum current that they can carry. IT DOES NOT
REPRESENT AN OVERLOAD TRIPPING CURRENT. The residual
current is expressed in Milli-amps and represents the
extent of current that can be lost from the system before
the device will trip.
WHILE AN RCD WILL PROVIDE A LEVEL
OF PROTECTION AGAINST ELECTRIC SHOCK IT SHOULD NOT BE
RELIED ON. THE SAFEST FORM OF PROTECTION WILL ALWAYS
BE AVOIDANCE
In Fig 1 the electrical load is properly contained
within an enclosure that is earthed. A person can not
come into contact with the live parts. The current flowing
towards the load “I” is the same as the
current flowing back from the load. Both currents pass
through the sensing element in the RCD cancelling out
to a net zero current.
In Fig 2 there is some breach in the electrical containment
allowing direct contact between the person and the electrical
live. A small current ‘dI’ flows through
the person to ground. Now the return current to the
RCD is ‘I – dI’ so the net current
flowing through the sensing element is no longer zero.
When ‘dI’ exceeds the tripping current the
RCD disconnects the supply.
Residual Current Breaker with Overload (RCBO)
These are devices that combine the functions of MCB
and RCD in a single Device
Add-On RCD
These are RCD’s that can not be used as stand
alone protection devices but are attached mechanically
to MCB’s or installed onto MCCB’s to create
an RCBO
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Two Pole and 4 Pole MCB/Vigi |
3 Pole MCCB/Vigi |
Shunt Trips & Volt Release
These are auxiliary devices that can be attached to
MCB’s or installed into MCCB units to arrange
for the tripping of an MCCB by remote signal from monitoring
equipment such as voltmeters, thermal trips, RCD current
transformers and emergency stop buttons etc.
An auxiliary that
clips to the side
of an MCB |
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An auxiliary that
fits inside an MCCB |
Ringmain units
These are devices for operation on High Voltage systems.
Options include
- A switch isolator
- An MCB
- An MCB and Switch Isolator
- Fuse monitoring
- Remote operation motorized control
We can supply High Voltage power transformers enclosed
in cubicles with the ringmain unit mounted on the enclosure
support skid. The unit is electrically close coupled
through the wall of the enclosure to the transformer.
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