Panel Meter / Electrical Indicator

What sets panel meters apart

The generator panel of a main switchboard is read every watch, often with a glance rather than a careful study. Panel meters give that overview: ammeters for load current, voltmeters for bus voltage, frequency meters, wattmeters for active power, and sometimes power-factor meters and synchroscopes for paralleling. Remote indicators repeat values from another location, for example engine speed or rudder angle on the bridge. Most panel meters do not measure the main current directly. They are fed through current and voltage transformers and scaled accordingly, which is why the scale on the dial must match the ratio of the transformer behind it.

Main designs

• Moving-coil instruments for DC and, with rectifiers, for AC measurements; accurate and readable, with a linear scale
• Moving-iron instruments for AC, rugged and common for ammeters and voltmeters in switchboards
• Electrodynamic instruments for power measurement and similar tasks
• Digital panel meters and multi-instruments that show several values on a display and often offer an output signal or a data link
• Stepping-motor analogue indicators and remote indicators, fed by a signal from a transmitter

Main components

• Measuring movement or electronic measuring circuit, with its scale or display
• Housing with a standardised square front, commonly in sizes such as 72, 96 or 144 mm, and clamps for panel mounting
• Scale plate with the printed range, unit and any extended scale for overload or motor starting
• Terminals, shunt resistors or transformer connections, depending on the measuring principle
• Optional lighting, so the dial is readable in a dim switchroom

Selection and sizing

• Measuring quantity and range, matched to the transformer ratio and the expected maximum value
• Accuracy class, which for indicators on a switchboard is usually coarser than for revenue meters
• Overload capability: ammeters for motor feeders need a scale that does not clip at the starting current
• Case size and cut-out, so the new instrument fits the existing panel without modification
• Supply for digital instruments, and the interface to the alarm or automation system if one is needed
• Environmental suitability: vibration and temperature range for a marine switchboard

Regulations and class

Class rules list the instruments that each generator panel and main busbar section must carry, and require them to be of an approved or tested type for marine use. Instruments connected to current transformers must be wired so that the transformer secondary is never left open under load: an open secondary circuit can develop dangerous voltage. Type approval is specific to the instrument series, and electronic multi-instruments in essential-service circuits are often covered by the class society's automation or electrical approval.

Typical faults

• Pointer sticking or dragging from dust, a bent pointer or a worn bearing in the movement
• Zero drift or an offset reading, usually after shock or ageing of the movement
• Wrong scale for the transformer ratio after a replacement, so the reading is correct in shape but wrong in value
• Internal shunt or series resistor overheating, particularly on instruments with overload exposure
• Display failure or blanking on digital units after a voltage spike or a failed supply
• Loose or oxidised terminals, causing a reading that jumps with vibration

What to look for in a supplier

• Exact match of the range, scale, case size and transformer ratio to the instrument being replaced
• Approval documents that cover the series, with a marine environmental rating
• Option for a custom scale plate, so an existing ratio can be kept
• For digital units, information on the output, interface and supply, and a firmware or configuration backup
• Spare movements or replacement front assemblies for the older series, if the instrument is no longer in production

Before replacing an ammeter, short the current transformer secondary or isolate it properly. Never open the secondary circuit of a loaded current transformer to swap an instrument.