Note: since IEC62052-11 Ed.2 the name for the internal relay is supply control switch (SCS).
Usually in electricity meters the load switch is a latching relay.
A latching relay keeps its contact position indefinitely without power applied to the coil. The advantage is that the coil consumes power only for an instant moment while the relay is being switched, and the relay contacts retain this setting across a power outage.
It is located between the supply input and load output terminals of the energy meter.
The relay is able to make, carry and break all values of currents between the minimum switched current rating to the rated breaking current for all values of the rated operating voltage range and the specified operating temperature range of the meter.
For prepayment meters (see also IEC 62055-31)
The rated breaking current (Ic) shall be equal to Imax of the payment meter.
The minimum switched current shall be equal to the nominal starting current of the payment
The rated breaking voltage (Uc) shall be equal to the upper limit of the extended operating voltage range of the payment meter.
The load switches have different categories for utilization. (UC = Utilization Category)
The payment meter load switching utilization category is subject to the purchase agreement between the payment meter supplier and the purchaser and shall be marked on the label of the payment meter as UC1, UC2, UC3, or UC4.
Category UC1 is applicable to payment meters rated at maximum currents up to 100 A. There is no requirement for the load switch to also switch the neutral circuit. The short time overcurrent is acc. to IEC 62053-21 for meters for direct connection (30* Imax for half cycle).
Payment meters with load switching category UC2, UC3 or UC4 shall have the following properties:
a) capable of making and breaking negligible currents of specified values
b) capable of making, breaking and carrying rated currents of specified values
c) capable of making into fault currents with specified value and under specified conditions
d) capable of carrying short-circuit currents of specified value for a specified time period and under specified conditions
e) not required to provide safety isolation properties in the open contact position. These are requirements for the installation mains isolation switch
f) not required to break overload currents or short-circuit currents. These are requirements for fuses and circuit breakers that are normally used to protect the installation.
|Fault current making capacity, C.5||2.5 kA||3 kA||4.5 kA|
|Short-circuit current, C.6 test 1||4,5 kA||6 kA||10 kA|
|Short-circuit current, C.6 test 2||2.5 kA||3 kA||4,5 kA|
For detailed information refer to IEC 62055-31, annex C.
Thank you for reading.
Editor's note: This article was originally published in October 2019 and has been updated for comprehensiveness.