Ring Main Unit (RMU) and Feeder Pillar Cables – Survey Before Digging

Updated: 23 October 2026

What is a ring main unit and what is the RMU full form?

RMU stands for ring main unit. A ring main unit is a compact, enclosed piece of high-voltage switchgear, in Indian distribution usually rated for 11 kV, placed at points along an underground cable network. It joins the incoming and outgoing cables of a ring and feeds one or more distribution transformers.

The name comes from the ring main layout. Instead of a single radial feeder, the 11 kV cable runs in a loop between two sources or back to the same substation. If one section fails, the switches in the RMUs on either side open to isolate it, and supply is restored to the rest of the ring from the other direction. Most RMUs combine load-break switches for the ring cables with a circuit breaker or fuse-switch for the transformer connection. Insulation may be gas, solid or air, and many newer units are motorised so the utility can operate them remotely.

What does a feeder pillar do in the LT network?

In power distribution, a feeder pillar is an outdoor LT cabinet on a plinth, holding busbars and fuses or circuit breakers. It receives 415 V supply from a distribution transformer, either directly or through an LT panel, and divides it into several outgoing cables. Those cables run to smaller distribution boxes or straight to consumers as service cables.

Where an overhead network has an LT distribution box on a pole, an underground network has a feeder pillar on the ground. Utilities use different names – LT pillar, distribution pillar, mini pillar or service pillar – often for different sizes of the same idea. For how HT and LT cables differ in voltage and construction, see our guide to the difference between HT and LT cable.

Which kinds of equipment sites does a survey meet?

RMU site

A plinth with ring cables entering from two or more directions and a transformer feeder leaving, plus an earthing system around it.

Compact or package substation

One enclosure combining RMU, transformer and LT panel. Ring cables come in, and several LT cables leave towards feeder pillars.

Distribution transformer plinth

A ground-mounted transformer with a separate RMU and LT panel nearby, linked by short HT and LT cables.

Feeder pillar

A smaller plinth with one incoming LT cable and anywhere from a few to many outgoing distributor and service cables.

Why is the ground around an RMU so crowded?

Equipment sites are where cables meet, so congestion is built into them. Within a few metres of a single RMU or compact substation you may find:

• two or more 11 kV ring cables, plus spare ducts for future extensions;
• the distribution transformer cable, HT on one side and LT on the other;
• several large LT outgoing cables, sometimes run in parallel;
• earthing electrodes and buried earth strips around the plinth;
• street-light cables, telecom ducts and water service pipes sharing the same footpath corner.

Sites are also chosen for convenience – a road junction, a park edge, the corner of a footpath – which are exactly the places other utilities already use. Adding a new RMU cable or extending a ring therefore means digging in the most crowded patch of ground on the whole route.

Which terms appear on RMU and pillar drawings?

• Load-break switch (LBS): a switch that can open a circuit under normal load, used for ring cables.
• Vacuum circuit breaker (VCB): a breaker that interrupts fault current, often protecting the transformer feeder.
• Way: one switched cable connection; a three-way RMU has three.
• Cable termination: the fitting that connects a cable end to the RMU bushing or pillar busbar.
• RMU cable: any HT cable terminated on an RMU, whether ring or transformer feeder.
• Plinth and cable entry: the raised base and the openings or ducts through which cables enter it.
• Earth mat or earth pit: the buried earthing arrangement serving the equipment.

How is an RMU or feeder pillar site surveyed before digging?

A long route survey uses scan lines at intervals along the road. An equipment site needs a different approach: a tight grid of close-spaced lines over the whole area of the plinth, cable approaches and earthing, so cables running in different directions all show up. On very congested sites, a 3D GPR survey rather than 2D lines can help separate them.

Ground penetrating radar records each pipe, duct and cable as a reflection, while an EM cable locator follows live cables and tells apart conductors that leave the unit side by side. At existing units, utility staff can sometimes allow a signal to be applied to a cable so it can be traced on its own; that is done only under the utility's permission and safety rules. Where results remain uncertain, a trial pit confirms the critical point. Our pre-excavation utility scanning suits single sites, and the HT/LT cable route GPR survey covers the cables between them.

When should an equipment site be surveyed?

• Before siting. Check candidate plinth locations so the unit is not placed over a water main or duct bank.
• Before extending a ring. Find the existing ring cables and their entry sides before trenching a new cable towards the unit.
• Before earthing work. Earth pits and strips are driven or trenched around the plinth, often close to existing cables.
• Before replacing a unit. Map every cable entry so the new plinth and terminations line up.

On distribution projects, the power utilities page describes how DISCOM teams plan survey across whole networks.

What goes wrong when equipment sites are not surveyed?

The problems are predictable, and most of them are expensive.

• A plinth is cast over a water main or duct bank and has to be broken out and rebuilt elsewhere.
• An excavator cuts an existing ring cable while trenching towards the unit, tripping a section of the network.
• Earth pits are driven into an LT cable, leaving a hidden fault that appears weeks later.
• A new LT outgoing is laid across an old service cable, damaging supply to a house that was never on the plan.

Each of these brings repair costs, outages and disputes over responsibility. A short survey before the work is a small fraction of that cost.

What should be recorded at each RMU and pillar?

Simple site records save a great deal of time later. For each unit, keep:

• the cable entering on each side, where it comes from and its approximate depth;
• the route of the transformer cable, HT and LT;
• each LT outgoing circuit and the streets or buildings it serves;
• the earthing layout, with pit positions;
• any spare ducts and where they lead.

A single sketch with offsets from fixed features such as kerbs, poles and building corners is often enough, and it should be updated whenever a cable is added or diverted. Mezux Touch has no affiliation with any distribution utility, and site surveys are done for whichever contractor or owner asks for them.

Planning work at an RMU or feeder pillar?

Mezux Touch maps equipment sites and the cable routes between them with its own GPR systems, EM locators and in-house crews from Faridabad. Charges are per metre of scanning: ₹24 for standard survey, or ₹32 when an L-section profile is wanted. Site surveys are usually ordered with the connecting routes, since every order carries a 4 km minimum, and work outside Delhi NCR has a ₹1,00,000 per-project floor. GST is added, and mobilisation, night shifts, traffic arrangements and drawings are quoted separately. Send your list of sites and routes through the quote form below.