Voltage Detection & Indication Systems for Medium Voltage Switchgear

The Challenge

Before any switching or grounding operation on medium voltage equipment, operators must verify the presence or absence of voltage on each phase. This is not an optional step; it is a safety-critical requirement mandated by both international standards and Turkish grid operator specifications. Selecting the correct capacitive voltage indicator system, however, is more complex than it appears. The indicator, coupling capacitor (C1) and connecting cable must be matched as a system to the switchgear nominal voltage. TEDAS and TEIAS project specifications define strict requirements on interface type and contact output configuration. Furthermore, the applicable standard has evolved from IEC 61243-5 to IEC 62271-213:2021, creating confusion when interpreting older tender documents.

Our Approach

How It Works

VDIS is a three-component system, not just the indicator. All three components must be matched and supplied together. The capacitive divider (C1) is permanently fitted on the busbar inside the switchgear panel and factory-mounted per specification. It is fitted on the busbar, not on the cable. The connection cable carries the low-energy capacitive signal from the divider to the indicator. Cable length affects the C2 calculation but there is no hard distance limit. The indicator unit (C2) is an LCD or LED display mounted on the panel door or front face. The divider couples a signal proportional to the line voltage to the indicator with no manual testing required, no contact with the primary conductor, and no external power supply needed for basic indication.

Applications

• MV switchgear panels
• Ring main units (RMUs)
• Transformer bays (Trafo Merkezi)
• Outdoor switchyards
• Any location requiring de-energisation verification before operator access
• Most common use: interlocking to prevent the earthing switch from closing while the busbar is live

IEC 61243-5, IEC 62271-213 and TEDAS Compliance

Three standards govern VDIS. IEC 61243-5:1997 is the original voltage detection standard still referenced in older TEDAS specifications. IEC 62271-206:2011 covers VPIS integration in switchgear. IEC 62271-213:2021 is the current standard for VDIS with a standardised LRM interface, and all Orion EE indicators comply with this. TEDAS-MLZ/95-007 clause 2.3 accepts both LED and LCD indicators. TEDAS-MLZ/2006-002 clause 8.10 requires LCD only, 1/8 DIN dimensions (45x92mm), NO+NC dry contacts, IP51 front panel protection, and surge protection. IEC indication thresholds: below 10% of rated voltage the indicator is off (safe); above 45% it must react (energised); the 10 to 45% range is intentionally left undefined by the standard. Thresholds are measured single-phase against the 3-phase rated voltage. A failed phase can carry induced voltage from the healthy phases that is sufficient to injure an operator but still below nominal level, which is the reason the 10% floor exists.

Selection Guide and Common Mistakes

Three parameters must be specified together when ordering: the C1 capacitance of the coupling electrode, the cable type and length, and the nominal voltage Un. The standard configuration in Turkey is C1=15pF, Un=36kV, C2=22nF. Custom C2 values are available from 1nF to 100nF in E10 steps. Mechanical strength of the capacitive divider depends on the installation type. Select 4 to 5 kN for fixed installations where no earth switch is connected to the divider. Select 10 kN when an earth switch is mechanically connected to and applies a pull force on the divider during switching. Selecting 4 to 5 kN in the latter scenario causes mechanical failure. Two common specification mistakes: the first is using rated voltage instead of operating voltage in C2 calculations — always use the actual service voltage. The second is mismatching the indicator interface during a replacement. Always verify that the interface type matches, not just the C2 value. IEC 62271-213 standardised the interface to LRM, but legacy installations may use earlier interface types that are not interchangeable.

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