{"id":2354,"date":"2026-09-26T09:00:00","date_gmt":"2026-09-26T01:00:00","guid":{"rendered":"https:\/\/shenhengglobal.com\/?p=2354"},"modified":"2026-09-24T18:14:58","modified_gmt":"2026-09-24T10:14:58","slug":"ring-main-unit-earthing-requirements","status":"publish","type":"post","link":"https:\/\/shenhengglobal.com\/vi\/blog\/ring-main-unit-earthing-requirements\/","title":{"rendered":"Y\u00eau c\u1ea7u v\u1ec1 n\u1ed1i \u0111\u1ea5t v\u00e0 k\u1ebft n\u1ed1i \u0111\u1ea7u cu\u1ed1i c\u1ee7a thi\u1ebft b\u1ecb ch\u00ednh"},"content":{"rendered":"
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ring main unit earthing<\/strong> needs a clear engineering boundary before it can support a purchase, inspection or commissioning decision. Ring main unit earthing is the controlled connection of the RMU enclosure, earth bar, cable screens, transformer or feeder earths and the site grounding system. It is not simply a green-and-yellow conductor attached to the cabinet. The arrangement must match the one-line diagram, the supplied SF6 RMU construction, the cable accessory design and the approved switching and safety procedure. A visible earth switch handle does not prove that every bonding path is intact.<\/p>\n

What must be bonded<\/h2>\n

Identify the main earth bar, enclosure panels, doors, removable covers, cable screen or sheath connections, transformer tank where applicable, surge arrester earths and the site earth grid connection. The exact list depends on the RMU configuration. The drawing should show the intended paths and any test links or removable bonds.<\/p>\n

Keep protective bonding separate from assumptions about neutral grounding. A cable screen may have a specific single-point or both-end bonding arrangement, while the enclosure requires a continuous protective path. Ask the supplier to mark the boundary and connection points on the as-built drawing.<\/p>\n

Earth switch and interlocking<\/h2>\n

The earth switch must operate only in the approved condition and must give a clear mechanical and electrical indication. Interlocks should prevent an impermissible operation, but an interlock is not a substitute for the site isolation, absence-of-voltage verification and grounding procedure. Test the exact RMU mechanism and record the result.<\/p>\n

Confirm that the earth switch is rated for the intended duty and that its position indicator agrees with the actual mechanism. Check key interlocks, trapped-key schemes, remote indications and local controls. Never force a handle or bypass a lock to complete a commissioning checklist.<\/p>\n

Cable screens and terminations<\/h2>\n

Cable screen connections should be mechanically secure, correctly identified and routed without sharp bends or unintended contact. Verify stress-control components, gland plates, screen earth leads, test points and the termination manufacturer instructions. A screen connection that looks complete may still be landed on the wrong feeder or left electrically discontinuous.<\/p>\n

When a cable withstand or diagnostic test is performed, coordinate the RMU boundary with the cable and accessory supplier. Isolate or protect surge arresters, voltage indicators and sensors as required by the approved method. After testing, discharge and re-ground the cable, restore every bond and cap, and independently check the final configuration.<\/p>\n

Continuity and resistance checks<\/h2>\n

Define the test boundary before measuring continuity or resistance. Record the instrument, lead arrangement, test current, points tested and environmental condition. The acceptance value belongs to the project specification, manufacturer instruction or applicable standard. Do not invent a universal ohmic limit because two RMUs have the same voltage rating.<\/p>\n

A four-wire low-resistance test may be useful for a main bond or joint, but it cannot reveal every hidden interface. Visual inspection, torque or connection records and comparison with the drawing remain necessary. If a reading is unstable, investigate contact pressure, paint, corrosion or parallel paths before accepting it.<\/p>\n

Earthing during maintenance<\/h2>\n

The operator needs a clear sequence for isolating the feeder, proving absence of voltage, closing the earth switch and applying any portable grounds required by the site procedure. The sequence must identify all sources, including backfeed, transformer LV paths, generators and adjacent circuits.<\/p>\n

Labels, mimic diagrams and remote status should be checked against the physical apparatus. A remote \u201cearthed\u201d indication is helpful but is not proof of a local safe state. Keep permits, locks, tags and independent checks aligned with the switching program.<\/p>\n

Fault duty and touch voltage<\/h2>\n

The site grounding study determines how fault current divides through the earth grid, cable screens, enclosure and other metallic paths. Review touch and step voltage, conductor thermal duty, earth grid connections and the clearing time used in the study. The RMU earth bar should not be selected in isolation from the whole installation.<\/p>\n

If the network fault level, clearing time or transformer connection changes, revisit the earthing design. A larger conductor alone may not solve a touch-voltage issue if the grid geometry or bonding path is wrong. Keep the approved study and equipment data together.<\/p>\n

Define the equipment boundary<\/h2>\n

Start with the exact asset, voltage class, construction and released drawing. A useful engineering answer is tied to a physical boundary and a defined operating state. Record where the inspection or calculation starts and ends, and identify equipment intentionally excluded. This prevents a general catalog description from being mistaken for a project acceptance value.<\/p>\n

Keep the product identity consistent through design, procurement and commissioning. Note the supplied model, accessories, enclosure, cable interface and environmental assumptions. If the installed arrangement differs from the drawing, stop and obtain an approved disposition instead of filling a missing value from another product family.<\/p>\n

Use standards and manufacturer evidence together<\/h2>\n

Standards define the framework, test concepts and terminology, while manufacturer documents define the verified construction and operating limits of the supplied equipment. A standard link cannot prove that an unverified modification is acceptable, and a brochure cannot replace the project specification or safety rules.<\/p>\n

For procurement, request the standard edition, design verification or type-test scope, routine test records, installation limits and any altitude or environment correction. Store the evidence with the asset record. When a result is disputed, compare definitions, test conditions and boundaries before comparing numbers.<\/p>\n

Plan installation and commissioning<\/h2>\n

Installation quality often determines whether a sound design remains sound in the field. Confirm foundations, access, lifting, clearances, ventilation, cable bend radius, earthing, labels and control interfaces before energization. Use the approved method statement and permit system. A neat cabinet is not proof that every bond, interlock or sensor is connected correctly.<\/p>\n

Commissioning should move from document checks to visual inspection, mechanical operation, electrical continuity, functional logic, protection or control tests and final restoration. Record instrument identity, settings, raw values, exceptions and the person who performed the check. Keep equipment isolated whenever a test or inspection exposes a hazard.<\/p>\n

Control changes and handover<\/h2>\n

A change to a barrier, cable route, relay setting, tap position, cooling path or earthing connection can alter the original design basis. Use a controlled change process with an owner, technical review, drawing revision and acceptance evidence. Temporary test arrangements must be removed or clearly tagged before handover.<\/p>\n

The final package should include the as-built drawings, test sheets, approved settings, equipment identity, maintenance instructions, deviations and training records. A future operator should be able to reproduce the decision without relying on memory or an unlabelled photograph.<\/p>\n

Safety note<\/h2>\n

Electrical equipment can retain energy and can be energized from more than one source. This article is not a switching instruction. Qualified personnel must follow local regulations, isolation, absence-of-voltage verification, grounding and supervision requirements. Never reach into an energized compartment or defeat an interlock to make a measurement.<\/p>\n

If a test fails, an enclosure is damaged, abnormal noise or discharge is observed, or the actual configuration is unclear, keep the asset in a safe state and escalate to the responsible engineer. Repeated energization without diagnosis can increase damage and risk.<\/p>\n

\"Ring
Use the approved product drawing and site conditions when applying this guidance.<\/figcaption><\/figure>\n

How to use the result in a project<\/h2>\n

Start with the released design and final equipment identity. Note the operating state, measurement point, environmental conditions and decision the evidence must support. Then compare the record with the applicable standard, manufacturer instruction and project acceptance plan. If a prerequisite is missing, keep the item open rather than filling the gap with a generic value. This gives procurement, commissioning and operations teams the same reference point.<\/p>\n

For a supplier inquiry, ask for the calculation basis, verified arrangement, limits, test records and deviations. For site work, record the person, instrument, date, isolation boundary and restoration check. A concise record is more useful than a long report that hides the one fact needed to approve the next step.<\/p>\n

\"Ring
Field verification should document the actual configuration, not only a general catalog illustration.<\/figcaption><\/figure>\n
\n\n\n\n\n\n\n\n\n\n
Project checks for ring main unit earthing<\/caption>\n
Check<\/th>\nEvidence<\/th>\nDecision use<\/th>\n<\/tr>\n<\/thead>\n
Definition<\/td>\nBoundary, equipment identity and operating state<\/td>\nPrevents different teams comparing different quantities.<\/td>\n<\/tr>\n
Design basis<\/td>\nRatings, environment and duty assumptions<\/td>\nConnects the answer to the real installation.<\/td>\n<\/tr>\n
Verification<\/td>\nApproved method, instrument, raw result and acceptance basis<\/td>\nCreates an auditable record.<\/td>\n<\/tr>\n
Change control<\/td>\nDeviation, review owner and as-built revision<\/td>\nStops hidden field changes becoming risk.<\/td>\n<\/tr>\n
Handover<\/td>\nSettings, tests, photos, maintenance and exceptions<\/td>\nSupports safe future operation.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

Related product and engineering guides<\/h2>\n

For the matching Shenheng product context, see the relevant product page<\/a>. For the broader topic, read the parent engineering guide<\/a>. Related reading: first related guide<\/a> and second related guide<\/a>.<\/p>\n

Sources and standards<\/h2>\n

Authority source 1<\/a> \u00b7 Authority source 2<\/a><\/p>\n

Video for system context<\/h2>\n

This neutral Practical Engineering explainer gives system context for substations, transformers and switchgear. It is educational background, not a switching or test instruction.<\/p>\n