Systems for measurement and control

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Live working: IEC/EN 61243-5

Chapter: Voltage Detecting Systems · System definition · Security advice

Capacitive voltage detecting systems

Regarding modern enclosed switchgears in the medium-high voltage range it is not possible to determine the voltage and phase conditions straight on the live parts, because these are no longer accessible.

Corresponding switchgears therefore provide standardized interfaces (coupling systems) for a connection with measuring devices according to IEC/EN 61243-5.

Block diagram coupling system

In principle capacitive dividers form these coupling systems (see figure below), composed by the coupling dielectric (in general integrated in insulators or cable test points) and the capacity of the coaxial connecting lead (cylindrical capacitor). Additional capacitances can be integrated where short connecting leads or single-lead wiring are concerned.

By connecting a device this voltage divider is loaded with a well-known capacitance (input impedance of the device). The current through this capacitance is directly proportional to the operating voltage of the switchgear and can therefore be used to determine her state (Uop = k * Imet, k = const.).

The constant k is chosen differently for each coupling system in order to ensure the correlation between a current of >2,5 µA and the state voltage present. In the case of cable test points (LRP) a current >1,0 µA is sufficient.

Operating conditions

Because the threshold value defined by a current of 2,5 µA, (or 1,0 µA) is independent of the voltage range of the switchgear, the constant k has to correlate to a certain percentage of the nominal voltage of the switchgear. In the reviewed standard the following definition were stated, here only quoted for the most common


The indication “voltage present” shall appear when the actual line-to-earth voltage is in the range of 45 - 120% of the nominal voltage, and shall not appear when the actual line-to-earth voltage is less than 10% of the nominal voltage. UNominal/Square root of 3 is the “line-to-earth nominal voltage”.

This correlation is presented in the drawing below. The diagram shows as y-coordinate the normalized operating voltage (fraction of the nominal voltage) versus the current out of the string part. The scale for the LRP system is given in the shaded area.

(U/I diagram)

The pairs of varieties (percentile voltage vs. current) located in the soft yellow area should not occur (=string part defective) because either the condition "voltage absent" for U < 10% (lower line through origin) or "voltage present" for U > 45% (higher line through origin) is not kept.


Definition of the coupling systems

Using the 2,5 µA interface current criterion given above the following coupling systems sufficient to the necessary input-resistance were defined:

System threshold value connecting system
HR (high resistance) 70-90 V / 36-43,2 MΩ 19 mm bipolar
MR (medium resistance) 20-30 V / 12-14,4 MΩ 21 mm bipolar
LR (low resistance) 4-5 V / 2-2,4 MΩ 6,3 mm phone jack
LRM (low resistance modified) 4-5 V / 2-2,4 MΩ 14 mm bipolar
LRP (low resistance for plug-type) 4-5 V / 5-6 MΩ (no plug defined)


Security advices

a) When the switchgear is operated below its nominal voltage a reliable indication of the voltage state cannot be guaranteed, because both current and voltage decrease. Thereby even a test of the interface in compliance with the 3,2 µA criterion could deliver wrong values as well as a voltage detector could maybe indicate “voltage absent” instead of “voltage present”. Therefore our products provide a residual-voltage-detection, actuated at a threshold of 0,5 µA. Furthermore the devices of the visual series can be tuned to the ration of operating/nominal voltage on-site and distribute thus a correct indication.

b) The LRM interface in not protected against polarity reversal. Please always take care of the correct wiring of the connection leads. Otherwise you could get wrong indications on commutated switchgears.

c) To HR string parts high voltages can be applied. Please pay attention to intact safety plugs and sockets.

d) Not automatically grounded LRP test points can hold high static voltage. Therefore flying sparks can occur in the moment of connection.