Start with the quantity being measured
An RMU may display current and voltage without providing the metering arrangement required for energy allocation or billing. Establish whether the project needs operating indication, transformer energy measurement, tenant allocation or an authority-approved revenue meter.
Mark the measurement point on the single-line diagram before selecting a module. A meter at a common bus measures a different boundary from one installed on a transformer feeder. The name “metering unit” does not determine which loads, losses or operating states are included.
This article concerns the electrical configuration inside an RMU assembly. Use the protection CT versus metering CT guide to distinguish secondary measurement duties, and record utility ownership and approval responsibilities separately.
Specify the measurement circuit as a complete chain
| Circuit item | What to define | Common ambiguity |
|---|---|---|
| CT primary location | The conductor and side of the switching device being measured | A shared bus measurement is mistaken for one feeder |
| CT secondary circuit | Ratio, core purpose, accuracy and connected burden | The meter is added to a protection circuit without review |
| VT connection | Measured phase quantities, ratio and winding duties | A voltage indication function is treated as a metering source |
| Meter inputs | Required current and voltage input characteristics | Instrument inputs do not match the offered transformers |
| Test facilities | Approved isolation and test-terminal arrangement | Testing would disturb an unrelated protection circuit |
| Data output | Required energy quantities and communication interface | Communications availability is mistaken for measurement accuracy |
Ask for both the primary diagram and the secondary metering schematic. A catalogue statement that CTs or VTs can be fitted is not enough to resolve the measurement chain.
Example: one RMU feeding two transformers
Assume the owner wants separate monthly energy records for two transformers. A single meter on the common incoming circuit gives an aggregate reading. It cannot directly provide both individual transformer readings, regardless of how many display pages its software offers.
The enquiry should identify separate measurement boundaries and show the circuits required to establish them. A different project may intentionally want only the site total. Neither arrangement is inherently better; the required energy account determines the equipment scope.
Check burden and available compartment space
Metering accuracy must be assessed with the proposed instrument-transformer characteristics and actual secondary circuit burden. Include the meter, test facilities and wiring. A long run to an external meter panel can change that circuit, even when the primary equipment stays the same.
Physical accommodation also matters. Ask for the position and orientation of CTs, VTs, primary protection where applicable, terminals and the meter compartment. Establish how authorized testing and replacement can be performed using the selected equipment's procedures. Do not assume that a side module, top module or integrated arrangement is available without a confirmed drawing.
Define acceptance records
A useful acceptance package identifies the actual instrument-transformer data, meter model, wiring revision and test results. Require checks that relate measured values to the correct phase and feeder designation. Record the final meter ratios and any configuration passwords or access responsibilities through the owner's approved handover process.
Send the marked measurement boundaries, system information, accuracy requirements, secondary lead lengths and meter specification through project enquiries. Use the ring main unit family to establish the assembly context. Jinxing should confirm the proposed metering arrangement and available evidence for that configuration; this guide neither establishes authority approval nor advertises standalone CT, VT or meter supply.