Replacing a single-phase meter with a three-phase whole-current meter at a shop
A shop owner in Pune has a sanctioned load of 10 kW. His existing single-phase meter must be replaced with a three-phase whole-current meter after he added two air-conditioners and a deep freezer. The supply is 415 V, three-phase, four-wire, 50 Hz. The utility issues a sealed three-phase four-wire static meter marked 10-60 A, 2400 imp/kWh, accuracy class 1.0, together with a sheet-metal meter box. The service cable is 4 mm2 copper, the run from the pole is 18 m, and the meter box must be fixed on an outside wall about 1.5 m above ground. The consumer's internal wiring is three-phase with a 32 A four-pole main MCB and a separate 6 A lighting MCB. Earthing is one old pipe electrode, measured at 8 ohm by the previous contractor using a multimeter between the earth pit and the neutral. The supervisor wants the job finished in a single visit, and the consumer wants supply restored before his evening rush.
1. Using the load figures, decide whether the issued meter can be connected directly or whether a CT-operated meter is required. Show the calculation and the checks you would make before leaving the store.
Read the worked answer
Current calculation
- Current per phase at unity power factor: I = P / (sqrt 3 x V) = 10 000 / (1.732 x 415) = 13.9 A.
- At a realistic 0.8 pf: I = 13.9 / 0.8 = 17.4 A.
The issued meter is a whole-current (direct-connected) meter with basic current 10 A and maximum current 60 A. The maximum demand of about 18 A is well below 60 A, and even a 100 percent overload would not exceed the meter range. So direct connection is correct and no CT is needed. A CT meter becomes necessary only when the load current exceeds the direct-connection range of the meter (typically above 60 A for this class of static meter); adding CTs also adds ratio error and a multiplying factor to every bill, which is unnecessary complexity here.
Checks before leaving the store
- Meter voltage rating matches 3 x 230/400 V, 50 Hz, three-phase four-wire.
- Accuracy class 1.0 is acceptable for whole-current consumer metering; the test certificate and calibration validity are inside date.
- Meter constant 2400 imp/kWh noted, so the pulse count can be verified on site.
- Seals intact, terminal cover present, and the terminal diagram printed inside the cover.
- Cable check: 4 mm2 copper carries roughly 25-30 A in conduit, so it is adequate for 17.4 A. Voltage drop is about 11 mV/A/m x 17.4 A x 18 m = 3.4 V, under 1 percent, which is acceptable.
- Main MCB rating 32 A is greater than the maximum demand but below the meter's 60 A maximum, so it protects the circuit.
2. Give the sequence you would follow from isolation to re-energising, explaining why each step matters, given that only one visit is allowed.
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Why the sequence matters: the meter is upstream of the consumer's main switch, so switching off the 32 A MCB does not make the meter terminals dead. Isolation must be at the utility's service cut-out or pole fuse.
- Pre-work briefing and PPE: insulated gloves rated for 1 kV, safety shoes, helmet; inform the consumer of the shutdown period.
- Isolate by pulling the service cut-out fuse (or the utility pole fuse) and lock/tag it. Keep the key/tag with you.
- Prove dead at the meter terminals with a two-pole voltage tester (test the tester on a known live point before and after).
- Remove the old meter and the box; fix the new box at 1.5 m, which is within the accessible 0.75-1.8 m band. Cable entry from the bottom with a gland so rain water cannot run in.
- Connect per the terminal diagram: R, Y, B and N incoming to the top terminals, outgoing load to the bottom terminals; neutral must not be fused or switched, and the outgoing neutral must be the same neutral, not a borrowed earth.
- Tighten terminals to the specified torque, tug each conductor, and check that no strand bridges two terminals.
- Insulation resistance test of the consumer's wiring at 500 V, with the meter disconnected or the meter electronics protected, expecting at least 1 Mohm.
- Seal the terminal cover and box with utility seals and record the seal numbers; get the consumer's signature.
- Re-energise and verify: 230 V phase-to-neutral on each phase, 400-415 V between phases, meter display on, no reverse-energy or error indication, and the pulse LED responding when a known load is switched on.
- Record the initial reading with date and time, hand over the installation certificate, and explain the reading to the consumer.
One-visit success depends on steps 1-6 of the first task being done at the store: a missing seal, terminal cover or test certificate forces a second trip.
3. The 8 ohm earth reading was obtained with a multimeter between the earth pit and the neutral. Is this acceptable for the installation? Explain what you would do before certifying the earthing.
Read the worked answer
The measurement itself is invalid. A multimeter between earth pit and neutral measures a loop that includes the neutral conductor, the transformer-neutral earth and the electrode under test; it tells you almost nothing about the electrode's own resistance, it can be distorted by stray DC/AC voltages, and it is unsafe because it requires touching an earth point and a neutral with meter probes. Earth resistance must be measured with a dedicated three-pole (or four-pole) earth tester using stakes driven at the specified spacing, with the electrode disconnected from the installation while testing.
Interpreting the 8 ohm value
- For a small consumer installation, a single electrode giving about 8 ohm is at the commonly accepted limit; many utilities prefer 5 ohm or lower where a second electrode can be driven. Treat 8 ohm as marginal but not automatically rejectable.
- The reading was taken in dry weather, which is the worst case; if a correctly measured value in dry weather is 8 ohm, monsoon values will be lower. The reverse situation (a low reading in wet weather) would be misleading.
What to do before certifying
- Re-measure with a proper earth tester, electrode isolated.
- Check the electrode is not corroded or broken at the top, and that the earth conductor from the pit to the meter box is continuous and firmly bolted.
- Bond the metal meter box to the installation earth.
- If the verified value exceeds the limit or the electrode is old, drive a second electrode about 3 m away and connect both in parallel, then re-measure.
- Do not earth the consumer's neutral at the premises as a shortcut; neutral earthing is done at the transformer, and bonding neutral to earth locally can cause dangerous circulating currents and false earth-fault paths.