Power monitoring and protective relays
Revenue and power-quality meters feeding building management and DCIM systems, and the protective relays that decide when a breaker opens.
What accuracy does metering need, and why is a relay setting never a local decision?
These two categories look similar and answer to very different requirements. Metering accuracy class governs what a reading may be used for -- revenue settlement demands far more than trend monitoring, and it is a commercial obligation rather than an engineering preference. Protective relays are not device-level settings at all: a relay is configured as part of a coordinated scheme, so changing one setting changes what happens everywhere downstream of it.
The 2 families in this category
24 documented configurations in total, with indicative lead times from 8 to 20 weeks. Each family links to its own reference page.
| Family | How it is specified | Lead time | Indicative band | Standards |
|---|---|---|---|---|
| Power Meters / BMS12 documented configurations | Class: Revenue / Power Quality Protocol: Modbus / BACnet / SNMP Mount: Panel / DIN |
~8 wk | $280 – $5K /ea | ANSI C12.20 IEC 61557-12 |
| Protective Relays12 documented configurations | Function: Feeder / Transformer Diff / Motor Comms: IEC 61850 / DNP3 Mount: Panel / Rack |
~20 wk | $1K – $14K /ea | IEEE C37.90 IEC 60255 |
What to settle in the specification
Accuracy class, matched to the use
Revenue metering is commonly specified to ANSI C12.20 accuracy class 0.2 or 0.5. Trend monitoring for a BMS or DCIM can accept less. The class sets what a reading can legitimately be used to settle, so it comes from the contract or the utility, not from preference.
The instrument transformers too
A meter is only as accurate as the CTs and VTs feeding it. Their accuracy class (IEEE C57.13) and burden have to suit the meter, or a class 0.2 meter reports class-1 numbers.
Power-quality capability
If the meter must report harmonics, sags and swells, specify the measurement method as well as the features. IEC 61000-4-30 Class A defines how power-quality quantities are measured, and IEEE 519 sets the harmonic limits usually being checked at the point of common coupling (IEEE 519 explained).
Relay functions and who owns the settings
Relays are specified by protective function, named by ANSI device number (51 time overcurrent, 87 differential, 27/59 under- and overvoltage). The settings come from the coordination study, not the installer, and are verified by acceptance testing to ANSI/NETA ATS.
Communications and time sync
State the protocol each device must speak (Modbus, BACnet or SNMP for meters; IEC 61850 or DNP3 for relays) and how devices are time-synchronized. Event records from different devices can only be compared if they share a clock.
Common questions
What accuracy class does revenue metering need?
Revenue metering is commonly specified to ANSI C12.20 accuracy class 0.2 or 0.5, with the class set by the utility or the contract the reading settles. The CTs and VTs feeding the meter need a matching accuracy class, or they limit the result.
What is the difference between a power meter and a protective relay?
A power meter measures and reports: energy, demand and power quality. A protective relay decides: it watches current and voltage and trips a breaker when a fault condition is detected. Relay settings come from a coordination study, because each one affects what happens downstream.
What do relay device numbers like 50/51 and 87 mean?
They are ANSI/IEEE C37.2 device function numbers. 50 is instantaneous overcurrent, 51 is time overcurrent, 87 is differential, 27 is undervoltage and 59 is overvoltage. A relay specification lists the functions it must provide by these numbers.