Metering · 7 min read

How to choose a multifunction meter for an LT panel

Six decisions decide whether the meter is useful or just a lit display. None of them are about the brand.

1. Start with why you are measuring

There are three honest reasons to put a meter in a panel, and they lead to different products. The first is operational visibility — is the feeder loaded, is the voltage sane, is the PF drifting. The second is cost allocation — how many units did the utility block consume last month against production. The third is billing, where somebody pays somebody else based on that number.

Only the third needs to survive an argument. Buying billing-grade accuracy for the first case is wasted money; buying a basic meter for the third case creates a dispute you cannot win.

2. Accuracy class

The class in the datasheet is the meter alone. Your installed accuracy is the meter plus the CT plus the burden. A Class 0.2S meter behind an ordinary Class 1 CT gives Class 1 results at Class 0.2S prices — a mistake we see in tenders every few months.

3. The CT decides your real accuracy

Size the CT primary to the actual maximum load current, not the incoming breaker. If a feeder peaks at 90 A, a 400/5 CT spends its whole life in the least accurate 20% of its range, and the error at 20 A is far worse than the datasheet suggests. Prefer a 100/5 and accept that you have no headroom for a load you are not going to add.

Then check burden in VA against the cable run from CT to meter. A long run through a large panel adds resistance the CT has to drive; undersized burden shows up as a low reading that nobody can explain. Where sub-metering must be added on a live plant, split-core CTs let you clamp over the existing cable without a shutdown.

4. Cut-out, depth and display

Measure before you order. 96×96 mm is the standard industrial cut-out and gives the largest display; 72×72 suits crowded doors; DIN-rail meters are the right answer for retrofit sub-metering inside a distribution board. Check depth behind the door too — a meter that fouls a busbar shroud is a costly surprise on commissioning day. And if operators read the panel from three metres away in bad light, choose the bright LED model over the elegant LCD.

5. Parameters you will actually use

Every MFM shows V, A, kW, kVA, kVAr, PF, frequency and kWh. The decisions worth making are about the extras:

• Maximum demand — needed if you are managing a sanctioned load or a demand-based tariff.
• THD and harmonics — worth having on incomers and on any board feeding many drives; it is the data that decides whether your capacitor bank needs detuning.
• Logging and time-of-use — only if somebody will read the logs. Otherwise a networked meter with software beats an on-board log nobody downloads.
• Alarm relay — cheap insurance for over-voltage, under-voltage or demand overshoot.

6. Communication

RS-485 with Modbus RTU is the practical default: cheap, robust, and readable by any SCADA, energy software or PLC/HMI. Daisy-chain the meters, terminate the line properly, and keep the cable away from drive output cables. Pulse output is a valid fallback for simple energy counting, and no communication at all is a fine choice for a local indication meter — as long as nobody promises a report from it later.

7. Five installation mistakes we keep finding

1. CT polarity reversed — shows up as negative kW or an impossible power factor on one phase.
2. Phase mismatch — the R-phase CT wired against the Y-phase voltage input; totals look plausible, per-phase values do not.
3. CT secondary left open — dangerous, and destroys the CT.
4. CT ratio not programmed — meter reads 5 A instead of 500 A; the commonest single fault we are called about.
5. RS-485 sharing a cable tray with drive outputs — communication drops that get blamed on the meter for years.

Quick answers

Is Class 0.2S worth the extra cost?+

Only for billing between parties or statutory reporting — and only if the CT class matches. Otherwise you are paying for accuracy the CT throws away.

Can one meter cover several feeders?+

A standard MFM measures one three-phase circuit. For several feeders, use purpose-built multi-circuit meters or one DIN-rail meter per feeder on a shared RS-485 loop.

Why is my power factor reading wrong?+

Nine times out of ten, CT polarity or phase mapping. Check that each CT sits on the same phase as its voltage input, and swap the secondary leads on the suspect phase.