Earth leakage monitor with alarm and trip

Paul Boughton

The FDB-VDA2 – from FDB Electrical Ltd – is a variable earth leakage monitoring device with alarm and trip for use with machinery incorporating motors or other equipment with inherent leakage characteristics and incorporates an adjustment to compensate for this variable. It is suitable for panel builders and plant maintenance engineers and so is likely to be of interest to designers/specifiers, installers, original equipment manufacturers, service engineers and machine manufacturers.

The FDB-VDA2 has loss of supply protection when its control power is lost the VDA2 will trip on re-connection indicating a power failure has occurred. When specifying in a distribution system do not use on primary breakers other NFS relays in the FDB range should be specified.

The FDB-VDA2 incorporates an auto-reset (can be disabled) which is especially valuable for remote equipment such as telemetry seismological monitoring stations. This permits automatic restart in the event that the earth fault self-rectifies and so saves on spurious service costs. The function is as a relay, for example in the FDB12 to replace the fixed rating relay fitted as standard. The relay trips an associated circuit breaker or contactor placed in series with the trip contacts.

In operation the leakage current is monitored by an external differential current transformer, eg MU28 which is connected via terminals to the VDA2. All conductors to be monitored except the earth wire are run through the current transformer. In a fault free healthy circuit the sum of all the current is zero.

In a fault situation current flows to earth which is detected by the current transformer and measured by the VDA2. Nominal control voltage is 24-230V AC/DC with tripping currents of 0.01A up to 10A and 0-10 Sec Time Delay.

A broken wire in the connection to the current transformer would disable this measurement enabling a protection circuit to detect this and so trip the VDA2.

The unit has two single pole changeover contacts: Terminals 11-12-14 for Trip; Terminals 21-22-24 for Alarm. Alarm is detected at 70 per cent of the selected trip level. The VDA2 is auto-reset and the trip is not stored. By fixing an external bridge connection across terminals X1 and X2 the trip is stored and has to be reset by pressing the reset button or removing the auxiliary supply. Adjustable delay uses the same setting for both alarm and trip.

To avoid any unauthorised adjustment of the setting potentiometers the unit has a transparent cover that could allow these to be sealed with a lacquer. Holes in cover above test/reset buttons allow activation. Power on indication is by green LED and alarm/trip indication is via red LED’s.

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