Earth leakage relays

The major components of earth leakage relays include a ZCT, amplifier, built-in relay, test button, earth leakage indication device.

  

Zero-phase current transformer (ZCT)

The ZCT is designed to detect the zero-phase current on a line and consists of a core made of a magnetic material with high magnetic permeability and a secondary winding on the core. In addition, it is provided with a shield case to prevent changes in sensitivity current under the influence of external magnetic field.

Amplifier

When ground fault current flows, electromotive force is generated on the secondary winding of ZCT. However, this electromotive force is very minute. The amplifier amplifies this minute signal and drives the built-in relay.

Built-in relay

The amplified signal level is compared by the comparison circuit in the amplifier, and, if the level is higher than the specified value, the coil of a small relay is excited. The relay has a built-in contact, which can retrieve an external signal when actuated. An alarm circuit or a breaking circuit can be configured with the aid of this contact.

Test button

The operation upon occurrence of earth leakage can be simulated and checked by pressing the test button. The button is intended to periodically check that the earthleakage relay can operate correctly.

Earth leakage indication device

When the earth leakage relay operates owing to ground fault, this device indicates the operation. An electric (LED) type and a mechanical (button) type are available. On the electric device, the lamp lights up when the relay operates. On the mechanical device, the indication button protrudes over the cover surface.

Connecting terminals

On the separate type, the relay and ZCT modules are separated. Therefore, it is necessary to electrically connect them. They are provided with connecting terminals.

Structure of each model

Mitsubishi earth leakage relays are classified into the following 6 types according to function and into two types of combination, a compatible type in which any relay and ZCT can be combined and an incompatible type in which the relay and ZCT with the same product number should be combined although the ZCTs of both types have the same appearance. The relay modules vary depending on the function.

1) NV-ZBA (small-size economical product)

This type has terminals for connecting the two terminals of ZCT to the ZCT terminals on the relay. The relay module has a sensitivity switching device, and the sensitivity can be switched in three stages. On the time delay type, not only the rated sensitivity current, but also the operating time can be switched in three stages. When a ground fault occurs, the relay will operate according to the signal from the ZCT, the built-in small-size relay will be driven, the signal at the relay output terminal will be switched, and simultaneously the earth leakage indicator lamp (LED) will light up (self-holding type). If the ground fault has been removed, the relay will return to the initial state by pressing the reset button. If the control power supply is connected to the load side of MCCB to break MCCB by the operation of the earth leakage relay, the reset operation is not required.

(2) NV-ZSA (general-purpose product)

Like NV-ZBA, this type consists of ZCT and relay modules. The releasing electromagnetic device in the relay module has been developed by Mitsubishi’s unique technology. Since it is a mechanical self-holding device, it can be reset by pressing the reset button serving also for indication of earth leakage. Like NV-ZBA, this model is characterized by the indication of earth leakage left even if the control power is turned off after it operates owing to earth leakage. It cannot be reset by turning off the control power.

(3) NV-ZHA (applicable to higher harmonics and surge)

The fundamental operation and structure of this model are the same as those of NV-ZBA. This earth leakage relay is provided with ICs applicable to higher harmonics and surge having an act ive f i l ter ci rcui t and a DPDC surge discriminating circuit. It can detect ground faults on the inverter secondary side to improve the performance of prevention of unnecessary operations caused by surge. This type complies with various standards (US UL Standard, Canada CSA Standard and European CE Marking).

(4) NV-ZLA (applicable to higher harmonics and surge)

The fundamental operation and structure of this model are the same as those of NV-ZSA. This earth leakage relay is provided with ICs applicable to higher harmonics and surge having an act ive f i l ter ci rcui t and a DPDC surge

discriminating circuit. It can detect ground faults on the inverter secondary side to improve the performance of prevention of unnecessary operations caused by surge. This type complies with various standards (US UL Standard, Canada CSA Standard and European CE Marking). It can be used also on 480V circuits.

Fig. 7. 21 Structure of earth leakage relay