Welcome to our e-rew Express. In our last issue, we discussed the common departures on metering installation requirements. This time, we would like to discuss the common departures on the customer low voltage main switch and interconnecting facilities requirements. We hope you can avoid these departures when you prepare your installation for connection of supply. We hope you will find the information of this e-rew Express useful and handy. If you have any suggestion, please send an email to us at mail@hec.com.hk or contact our Customer Installation Section at 2887 3455 so that we can further improve our service. Common Departures on Customer Low Voltage Main Switch and Interconnecting Facilities Requirements 1. Main Incomers i. Protection Scheme The time-current curve of overcurrent protection of customer main incomer directly connected to transformer cannot grade with HK Electric infeed protection. The type and setting of customer main incomer s protective devices shall be graded properly with HK Electric infeed protection.
ii. Switches Directly Connected to Transformer Main Incomer Essential Main Switch 35kA 45kA The short circuit breaking capacity of the switch directly connected to transformer is inadequate. Short circuit breaking capacity of the switch directly connected to transformer shall exceed 40kA. The switch directly connected transformer shall not be of fixed type. to Draw-out type or plug-in type circuit breaker shall be used. 2
iii. Undervoltage Protection Undervoltage Protective Device Undervoltage Protective Device Undervoltage protective devices should not be installed at main switches or switches affecting more than one customer / equipment. Undervoltage protective devices should be installed at the load side only in order to avoid unnecessary supply interruption to other equipment / customers. 2. Customer Main Switch i. Main Switch Position A customer main switch shall not be installed after HK Electric s meter. A customer main switch shall be installed and connected immediately before HK Electric s meter. 3
ii. Single Phase Installation A single pole switch cannot be used as a main switch for single phase installation. A main switch for single phase installation shall be of double-pole type interrupting live and neutral. iii. 3-phase Installation Neutral Link Neutral link in the neutral conductor of a 3-pole main switch for 3-phase installation has not been provided. A main switch for 3-phase installation shall be of 4-pole or TPN switch. If 3-pole linked switch or circuit breaker is used, a link shall be inserted in the neutral conductor and securely fixed by bolts or screws. iv. Temporary Installation or Overhead Line Supply A MCB shall not be used as the main switch for the installation which is supplied from an overhead line installation or for temporary installation. A RCD shall be used as the main switch for the installation which is supplied from an overhead line installation or for temporary installation. 4
3. Riser Switch Earth fault relay associated with protection C.T.s could be used to achieve automatic disconnection within 5 seconds during an earth fault. 4. An instantaneous trip RCD should not be used as riser switch or cutout of multi- customer premises. Interconnecting Circuit Breakers Between Two or More Supply Sources i. Section Switch Overcurrent protection of the section switch has not been provided. With Overcurrent Protection The section switch shall be provided with proper overcurrent protection. ii. Changeover Switch A 3-pole changeover switch shall not be used for interconnecting the normal and standby sources. 5 A mechanically and electrically interlocked 4-pole changeover switch should be used for interconnecting the normal and standby sources to ensure that neutral unbalance and fault currents return to the correct source of supply.
iii. LV Service Cables No interconnecting facilities between HK Electric s LV service cables are allowed. We are pleased to provide advisory services to REC/REW regarding the customer low voltage main switch and interconnecting facilities arrangement. Please provide us with the schematicc wiring diagram. We will study and feedback our comment or suggestion to you. 6