SMY133-SMY134-SMP133 Operating Manual
5. Residual Current Monitoring (RCM)
Instruments equipped with residual current inputs can be used for residual current monitoring (RCM).
The RCM can get early warning of measured network failures.
The instruments measure alternating and pulsating direct residual currents according the A-type RCM
specification as defined in the IEC 62020 standard. No directional sensitivity of the residual currents is
implemented.
5.1 Measuring Transformers for the RCM
All of the instruments are designed for indirect connection, thus use of a current transformer is
necessary. But for the RCM, special residual current transformer (RCT) must be used. The following
parameters are essential for a suitable RCT selection :
•
In
… rated current = maximum primary current of the RCT. The RCT
In
must be greater than
the upstream circuit beaker rating (= maximum permanent phase current of monitored
network). If not specified, the RCT
Icth
(rated continuous thermal current) can be used
instead.
Usually, size of the RCT and its window depends on the
In
value – check it too in order the
cable used to match it.
•
RCT-ratio
… (600/1, for example)
•
IΔn
… primary residual operating current. It defines range of primary residual currents that can
be measured with defined accuracy and used for reliable indication of a residual current
threshold exceeding. Then :
◦
desired residual current threshold must fit into the RCT
IΔn
range
◦
the desired residual current threshold divided by the RCT-ratio must fit into the
instrument residual current range
•
R
RCMMAX
… maximum burden = maximum impedance of the shunt connected to the RCT
output. It must be higher than the instrument residual current input impedance.
5.1.1 Electrical Safety
The residual current measurement inputs are not insulated from the instrument internal circuitry,
therefore signals of safe voltage only can be connected to.
Insulation of used RCT must fulfil IEC61010-1 requirements for the CATIII double insulation
for the mains voltage present!
5.1.2 Standard RCTs with AC-Current Output
These RCTs are commonly used. Appropriate
CT
RCM
–ratio in form xxx/1 must be set in the
Installation
group of parameters.
But it must be taken in account that in certain cases possible overcurrent on the RCT output could be
dangerous for the instrument input.
5.1.2.1 Overcurrent Protection
Unlike standard current inputs, the residual current inputs are designed to measure residual current in
range of milliamperes (see technical specifications), i.e. currents that are several orders of magnitude
35
