Vedic Pivot: Introduction to LV system,

LV System Introduction :

Below shown is the typical single line diagram (SLD) of LV system in industry. In most of the industry primary source of power supply is from electricity board (through transformer) & generator is used as standby power source in case supply from EB fails or power quality from EB is not in acceptable limits.

So, the separate switching & protection device is used for each source. As the fault levels at these locations are very high, protection device should have high breaking capacity & switching operations at these locations is very infrequent. Hence ACB is used at these locations. (Location: 11 & 12) Purpose of these breaker is to protect bus bar under it & provide isolation for whole system.

Generally, the capacity of generator is smaller than the transformer ration due to economic reasons, hence only circuit loads are operated when generator is providing the power. Please note the generator is used as stand by source, hence transformer and generator are not working in synchronism. If breaker 11 & 12 are closed simultaneously there can be circulating current following through them or in worst case there can be short circuit. To avoid this condition buscoupler (Tie breaker; breaker 13) is used. By this arrangement all the critical loads are connected to busbar under generator set and noncritical loads to busbar under transformer. When EB supply is healthy breaker 11 & 13 is closed and supply is given to all critical & noncritical loads. When generator is supplying the power the breaker 12 is closed and only critical loads will get supply. Generally interlocking system is used to ensure the same. Generally, ACB is used as bus coupler.

No. of outgoing feeder are taken from the busbar for various loads. Every feeder has its own switching and protection device. ACB/MCCB can be used for outgoing feeders. Purpose of these breakers is to protect the underground cable & give the backup protection to breaker 31. Panel consisting of incoming breaker, busbar, bus coupler and no of outgoing breaker is called as PCC panel (Power control center panel).

From outgoing feeder supply is given to loads through MCC panel. Every MCC panel has its own incoming breaker for switching, protection, & isolation purpose (breaker 31). Generally, MCCB/SDF is used at location 31. From MCC panel supply is given to various motors. For control and protection purpose every motor has motor feeder combination. Motor requires switching along with overload, single phasing & short circuit protection. In most of the cases switching of the motor are frequent, hence switching device should have high life. Generally switching device is contractor, which will give the highest operating life. For overload & single phasing protection of motor Bi-metal relay (BMR) is used. SDF/MCCB is used as SCPD (Short circuit protection device) & isolation purpose.

Relevant Standards:

IS/IEC: 60947

· Part 1 ---- General Rules for all products

· Part2 ---- Circuit breakers

· Part 3 ---- Switch disconnector Fuses

· Part 4 ---- Contactor/Relays/Starters

· Fuses ---- IS;13703/ IEC 60269
· MCB ---- IEC 60898

Switchgear Distinguishing Features

Type	Circuit Rating	Fault Operating Capacity	Frequency of Operation	Life expectancy (No. of Operations)
ACB	High	High	Infrequent	Low
BCCB	Medium	Medium	Infrequent	Low
FUSE	Medium/Low	High	Once	Once
SDFs	Medium/Low	High/Medium	Frequent	Medium
CONTACTORs	Low	Low	Very Frequent	High