10.5 Metering Devices & Regulating Apparatus

Metering Devices

Domain III subdomain K covers metering devices: watt-hour meters, ammeters, voltmeters, power and power-factor meters, transducers, and the digital multifunction meters that have replaced most of them. The Level 2 DCO asks you to identify properties, types, and applications, apply visual and mechanical inspections, employ methods and procedures for electrical tests, and evaluate test results.

The Chain That Determines Accuracy

A meter reading is the product of an entire measurement chain, and the meter itself is rarely the weak link:

Primary current/voltage -> CT/PT -> secondary wiring -> burden -> meter

An error anywhere multiplies through. This is why metering verification always starts upstream of the meter:

• CT and PT ratio — verify the actual ratio and, on multi-ratio CTs, the tap actually landed. A 1200:5 CT wired on its 600:5 tap while the meter is programmed for 1200:5 reports exactly half the true current.
• Polarity — a reversed CT reverses the sign of real power on that phase. On a three-phase meter, one reversed CT does not simply subtract that phase; it produces a reading that can look plausible but is badly wrong, and it flips the sign of kW and kVAR contributions.
• Burden — the total VA of the connected secondary circuit, including the meter, wiring, and any shunts. Exceeding the CT's rated burden pushes it out of its accuracy class. Long secondary runs of small conductor are the usual cause.
• Secondary wiring and shorting screws — verify continuity, correct phase assignment, and that CT shorting blocks are open when in service and closed before you lift a lead.
• Meter programming — CT and PT ratios, wiring configuration (4-wire wye, 3-wire delta), and pulse constants must match the installation.

Accuracy Classes

Instrument transformer accuracy is specified at a stated burden, not in the abstract. Under ANSI/IEEE C57.13 practice, metering-class accuracy is far tighter than relaying-class: metering CTs are typically held to a fraction of a percent ratio error at their rated burden, while relaying accuracy classes (C-class ratings such as C400) specify that ratio error stays within about 10 percent up to 20 times rated current into a stated burden. That difference is the exam point: a relaying CT is not a metering CT. Using one for revenue metering produces legally defective billing; using a metering CT for protection produces a core that saturates during faults exactly when the relay needs a faithful signal.

Revenue metering accuracy classes are defined under the ANSI C12 metering standards, with the tighter classes reserved for billing applications.

Visual, Mechanical, and Electrical Tests

• Inspect for physical damage, correct mounting, seals intact on revenue meters, and legible nameplate and multiplier data.
• Verify the meter multiplier posted on the enclosure matches the installed CT and PT ratios; a wrong posted multiplier is a common and expensive finding.
• Verify the display, communications, demand reset, and time and date settings.
• Verify accuracy at multiple points — typically light load, full load, and at a lagging power factor — because a meter can be accurate at unity and wrong at 0.5 lagging if a current or voltage element is bad.
• Compare the meter's readings against a calibrated reference standard at the same instant, using the same instrument transformer secondaries.
• Where the meter reads through transducers, verify the transducer output span against injected inputs.

When a meter and a portable analyzer disagree, resolve it upstream: check the CT tap, then polarity, then burden, then the programmed ratios. Only after all four are verified is the meter itself the suspect.

Regulating Apparatus

Domain III subdomain L covers regulating apparatus: step-voltage regulators, load tap changers (LTCs), and induction regulators. At Level 2 the DCO lists identify properties, types, and applications, and apply visual and mechanical inspections.

How a Step-Voltage Regulator Works

A step-voltage regulator is an autotransformer with a tapped series winding and a motor-driven tap changer, plus a control that senses output voltage and drives the taps to hold a setpoint. The standard is plus or minus 10 percent regulation in 32 steps — 16 raise and 16 lower — which gives 5/8 of one percent (0.625%) per step. A reversing switch flips the series winding to change between raise and lower ranges, and the neutral position is where the series winding contributes nothing.

The control uses line-drop compensation — R and X settings that model the impedance of the line downstream — so the regulator holds voltage at a remote load center rather than at its own terminals. A regulator with wrong compensation settings will hold the wrong voltage at the customer, which is a common commissioning finding.

The Bypass Sequence That Matters

The series winding of a regulator carries the full line current. Opening it under load is like open-circuiting a current transformer secondary — the winding tries to sustain current through an open gap and produces a destructive, potentially lethal arc and voltage.

The rule set:

1. Drive the regulator to the neutral position first and confirm it there — both by the position indicator and, where equipped, by the neutral light.
2. Close the bypass switch to establish a parallel path for load current.
3. Only then open the source and load disconnects, isolating the regulator.
4. Reverse the order to restore: close the disconnects, verify neutral, then open the bypass.

A regulator that is not on neutral when bypassed places the series winding across the bypass and circulates a large current through it. "Neutral first, bypass second, disconnects last" is worth memorizing verbatim.

Inspections and Tests

The LTC-oil point is worth carrying: on a transformer with an LTC in its own compartment, a dissolved-gas result from the LTC compartment showing arcing gases is normal, because the tap changer arcs by design every time it operates. Confusing an LTC oil sample with a main-tank sample generates a false alarm — and mixing up the sample labels is a real and common field error.