Masterpact NW Restricted Earth Fault Protection Date: November 2011 Page 1 of 12
Contents Introduction 3 Principle of Operation 4 Equipment Required 5 Installation of the Neutral CT 5 Connection of the Neutral CT 6 24V DC power supply 6 Programming the M2C Contact with the Micrologic 6.0E 7 & 8 Programming the M2C/M6C Contact with the Micrologic 6.0P/6.0H 9 Resetting the ACB after a Fault 10 Resetting the M2C/M6C after a Fault 10 Test Procedure 11 & 12 For further information please consult our technical support department on 0870 608 8 608 Date: November 2011 Page 2 of 12
Introduction Restricted earth fault protection provides earth fault protection for the secondary side of the transformer and the cables/busbars up to the incoming terminals of the LV incoming circuit breaker. This type of fault occurs upstream of the incoming circuit breaker and therefore can only be disconnected by the MV circuit breaker on the primary side of the transformer. The Masterpact NW 3 or 4 pole ACB can be used to detect restricted earth faults. The ACB uses its internal CT s on phases L1, L2, L3, external neutral CT and an appropriate Micrologic trip unit to monitor the system. The neutral CT should be fitted as close to the ACB as possible. The neutral to earth link is connected upstream of the neutral CT as shown in Fig 1. Fig 1: Restricted Zone Unrestricted Zone L3 L2 L1 N Ext CT ACB N/E Link Date: November 2011 Page 3 of 12
Principle of Operation In the event of an earth fault upstream of the Masterpact ACB, the fault current will flow from the source of the fault, to earth. The earth fault current will then return back to star point of the transformer via the neutral earth link, external neutral CT and neutral cable/busbar. Earth fault currents upstream of the ACB are not detected by the ACB s internal CT s on L1, L2 or L3. However, the earth fault current returns to the star point of the transformer via the neutral earth link and external neutral CT. The Micrologic trip unit carries out a summation calculation of the currents in all of the CT s (L1, L2, L3 & external neutral CT). If the resultant summated current is greater than the earth fault setting on the Micrologic trip unit, the ACB will trip. When the ACB trips on an earth fault, the Micrologic trip unit will signal the programmable M2C or M6C contact to operate. This contact is used to trip the MV circuit breaker via a tripping coil on the primary side of the transformer. When the earth fault has been rectified, the M2C or M6C contact is reset via the Micrologic trip unit, along with the red reset button on the front of the ACB. It will then be possible to re-close the MV circuit breaker if it s safe to do so. Figure 2 shows the LV side of the transformer and an example of an earth fault upstream of the ACB. Fig 2: Date: November 2011 Page 4 of 12
Equipment Required Restricted earth fault protection can be achieved using either a 3 or 4 pole Masterpact NW ACB which must be equipped with the following parts. Micrologic 6.0E*, 6.0P** or 6.0H** trip unit Programmable M2C or M6C contacts External Neutral CT for 3 & 4 pole devices 24V DC Power supply module Battery module*** * The Micrologic 6.0E only supports the M2C contacts, which can only be programmed for non-latching control when used with the Micrologic 6.0E. ** The Micrologic 6.0P or 6.0H support either the M2C or M6C contacts, which can be programmed for either latching or non-latching control. *** The battery module is required if the M2C or M6C contacts are programmed for latched output. It s used to provide a back up to the 24V DC feed to the trip unit in the event of the power supply losing its input supply. If the 24V DC power supply is independently fed from a secure supply the battery module may not be required. Note: If a 4 pole Masterpact NW is used, please state at the time of ordering that it s going to be used on a restricted earth fault application. Installation of the Neutral CT The external neutral CT must be installed upstream of the neutral earth link for the restricted earth fault system to operate correctly. The length of the secondary wiring must not be greater than 4 metres. The polarity of the CT is vital. The orientation of the CT (H1 & H2) is always the same with respect to the ACB, irrespective of the direction of the power flow. This is because the orientation of the CT s inside the ACB are fixed, i.e. H1 connected to the bottom terminal and H2 connected to the top terminal. See figure 3. Fig 3: Date: November 2011 Page 5 of 12
Connection of the Neutral CT The secondary connections between the external CT and the Micrologic 6.0 trip unit are shown in figure 4. Fig 4: Notes: The maximum length of the secondary winding between the CT and the Micrologic trip unit is 4 metres. The dotted VN connection on the diagram is required to be connected when using a 3 pole ACB. The link & capacitor shown between terminals T1, T3 & T4 on the CT are supplied with the CT. 24V DC Power Supply A 24V DC power supply must be connected to terminals F1 & F2 on the Micrologic trip unit when the M2C or M6C contacts are used. Terminals F1 & F2 will automatically power the M2C contacts if fitted. If the M6C contacts are used the same power supply can be used to feed the Micrologic and the M6C contacts. The recommended 24V DC power supply can be found in the ACB catalogue. Date: November 2011 Page 6 of 12
Programming of the M2C contact with the Micrologic 6.0E The following Instructions show how to set up the M2C contact with the Micrologic 6.0E. The Isd setting shown in the example below should be replaced with the earth fault trip indication when used in a restricted earth fault application. Note: The text below refers to page 32 this information is on the next page of this document. Date: November 2011 Page 7 of 12
Information regarding page32 referred to on the previous page. Date: November 2011 Page 8 of 12
Programming of the M2C or M6C contact with the Micrologic 6.0P or 6.0H Note: Before programming the contacts the cover over the trip unit dials needs to be opened. The alarm type is selected from the alarm menu, which is accessed by pressing the spanner button on the trip unit as shown below. Any contact can be chosen and the alarm type is set to earth fault. When the alarm type has been confirmed, press the spanner button and select yes and then press the spanner button again to return to the previous menu. The contact is then set up to either latching or non-latching mode. When the alarm mode has been confirmed, press the spanner button and select yes and then press the spanner button again to return to the previous menu. If latching is selected the 24V DC power supply must be maintained to the Micrologic trip unit & M6C (if fitted). If the 24V DC supply is not maintained the alarm contact cannot be reset after tripping on an earth fault. Date: November 2011 Page 9 of 12
Resetting the Masterpact ACB & M2C or M6C contact after an earth fault trip When the reason for the ACB tripping after an earth fault has been identified and resolved, the ACB can be reset. If the M2C or M6C contact has been set to non-latching mode the red reset and LED reset buttons on the front of the ACB have to be reset as shown in step 1 below. If the M2C or M6C contact has been set to latching mode, the additional operation of resting the contact will also have to be carried out as shown in step 2 below. 1. Resetting the ACB Resetting the fault Indication LED 2. Resetting the M2C or M6C latching contact Date: November 2011 Page 10 of 12
Test Procedure When the Masterpact has been installed into the switchboard, it s necessary to carry out primary current injection tests to prove that the earth fault protection operates correctly. The tests detailed below will prove the following: The ACB trips at the required earth fault value set on the Micrologic trip unit. The external neutral CT has been fitted and wired correctly. The M2C or M6C contacts have been programmed correctly to operate when the ACB trips on an earth fault. Test Procedure (Quick Test) It s safe to assume that the CT s inside the circuit breaker are fitted & connected correctly. Therefore, the Quick Test can be used to prove that the connection and polarity of the Neutral CT is correct compared to the circuit breaker. 1. The Micrologic trip unit & M6C contact if fitted need to be supplied with 24V DC. 2. Set the earth fault element Ig on the trip unit to a suitable current, with the time delay Ih set to zero seconds. 3. Inject any one of the phases of the circuit breaker with current, slowly increasing it until the circuit breaker trips. The tripping current will be the E/F setting current +/- the published tolerance, as shown in diagram 1. The M2C Contact should operate each time the circuit breaker trips. 4. Inject the same phase, connected in series with the Neutral CT as shown in diagram 2. The test current should be about 20% higher than the previous tripping current and should be maintained for several seconds. The circuit breaker should not trip on earth fault if the Neutral CT has been fitted and wired correctly. Test Procedure (Full Test) It should only be necessary to carry out this test if there is a problem regarding the correct operation of the earth fault element on the circuit breaker. This test procedure will compare the polarity of any CT in the circuit breaker to any other CT in the circuit breaker. 1. The Micrologic trip unit & M6C contact if fitted need to be supplied with 24V DC. 2. Set the earth fault element Ig on the trip unit to a suitable current, with the time delay Ih set to zero seconds. 3. Inject any one of the phases of the circuit breaker with current, slowly increasing it until the circuit breaker trips. The tripping current will be the E/F setting current +/- the published tolerance, as shown in diagram 1. Repeat the test on the other two phases. The M2C Contact should operate each time the circuit breaker trips. 4. Inject a phase connected in series with the Neutral CT as shown in diagram 2. The test current should be about 20% higher than the previous tripping current and should be maintained for several seconds. The circuit breaker should not trip on earth fault if the Neutral CT has been fitted and wired correctly. Repeat for the other two phases. 5. Inject a phase connected in series with another phase i.e. L1 to L2 as shown in diagram 3. The test current should be about 20% higher than the previous tripping current and should be maintained for several seconds. The circuit breaker should not trip. Repeat for the other combinations i.e. L1 to L3 and L2 to L3. Date: November 2011 Page 11 of 12
Diagrams showing Primary Current Injection set (AC) connected to the power connections on the rear of the ACB/Chassis. Diagram 1: Diagram 2: Diagram 3: Notes: If the circuit breaker trips during the test shown in diagram No.2. Check the polarity of the CT, the secondary wiring and that the correct CT has been used. If the circuit breaker is equipped with an ammeter it will indicate the injected currents (check Micrologic instruction manual for minimum operating threshold). Date: November 2011 Page 12 of 12