The Ring Main Unit (RMU) is a type of switchgear used in the distribution networks that is normally formed by interconnecting a number of ring shaped grids. It is designed to operate at the rated nominal system voltage of 11kV and is normally consist of switches on both sides of the T-off, one to open the ring and another at the connection to the distribution transformer (usually rated 1000kVA) using one of the following:
Swith and fuse separately
Switch-fuse combination
Circuit breaker
Basic Construction Aspects of RMU
The equipment is metal enclosed, free standing or transformer mounted type (in DEWA, they call it Substation and Pocket types respectively)
Free standing RMU shall be stable and rigid on its own support
The unit is provided with lifting facilities of proven design for easy handling
The equipment is complete with necessary cable termination boxes, earth fault indicating device with current transformers, fuse switch and switching equipment with necessary interlocks for safety and designed for minimum maintenance.
The mechanical design and strength of the unit and components are able to bear the mechanical stresses on the switch terminals when installed and in operation
The equipment can withstand the electro-dynamic forces without reduction of the switches reliability or current carrying capacity.
Cable Terminations
The ring cable terminations are provided on the sides of the RMU. The T-off termination is provided on its rear in the case of free standing type. In the case of transformer mounted type, T-off connection is brought out in such a manner to connect direct to the transformer HT side.
The cable terminations is suitable to receive three core underground cables of sizes up to 240mm. sq. of XLPE Copper cables.
Operational Switches
RM Switches is composed of a bank of three positions (ON, OFF, EARTH) single break type suitable for the three phase current carrying capacity
Tee-off switch is composed of a bank of three positions (ON, OFF, EARTH) double-break type suitable for the phase current carrying capacity
Current limiting device is composed of the striker pin type and also fuse links capable to actuate a common tripping bar to protect the distribution transformer and to avoid chances of single phasing.
Quite a job eh!.. 'til next time.
