2.3 Surge, Overvoltage & Undervoltage Protection Devices

2.3 Surge, Overvoltage & Undervoltage Protection Devices

Quick Answer: Overcurrent devices protect against too much current. An entirely separate family of devices protects against the wrong voltage. Surge protective devices (SPDs) and surge arresters clamp microsecond transients from lightning and switching. Undervoltage, overvoltage, phase-loss, phase-reversal and voltage-unbalance relays protect against sustained or repetitive voltage faults that never draw enough current to trip a breaker but destroy motors, drives and control systems. The Red Seal Occupational Standard treats these as their own sub-tasks (B-8.03 through B-8.06) because they are their own engineering problem.

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1. The Transient Environment Inside an Industrial Plant

A modern plant generates most of its own surge damage. Direct lightning is spectacular but rare; the routine sources are internal:

The sheer repetition is what matters. A plant that switches a capacitor bank four times a shift delivers thousands of moderate transients a year into every rectifier front end on the bus. Each one takes a small bite out of semiconductor junctions and insulation. Equipment does not fail on the day it was damaged.

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2. Metal-Oxide Varistor Technology

Nearly every low-voltage SPD and every modern medium-voltage arrester is built on metal-oxide varistors (MOVs) — a sintered zinc-oxide ceramic whose resistance is extremely non-linear.

• Below its threshold, an MOV looks like an open circuit (microamps of leakage) and the system never knows it is there.
• Above its threshold, its resistance collapses by orders of magnitude and it shunts surge current to ground or to the neutral, clamping the voltage across the protected equipment.

Degradation and end of life

MOVs wear out. Each conduction event slightly alters the grain boundaries, and the leakage current at normal operating voltage creeps upward. Two failure paths follow:

1. Gradual degradation — leakage current rises, the disc self-heats, the threshold falls further, and the process accelerates toward thermal runaway.
2. Catastrophic failure — a surge beyond the device's energy rating punches through, and the MOV fails as a short circuit across the line.

This is why a proper industrial SPD is not just a block of MOVs. It contains:

• Internal thermal disconnects that open the MOV from the supply when the disc overheats, converting a potential fire into a failed-safe open circuit;
• Status indication — green/red indicators or dry contacts per phase, wired back to the PLC or building system so a failed module is reported rather than discovered;
• A surge counter on higher-end units so the maintenance record shows how hard the device has been working.

Maintenance reality check: an SPD with a dark or red status light is not protecting anything. It sits there looking installed. Checking SPD status indicators is a line item on every competent industrial preventive-maintenance route.

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3. SPD Types and Where They Are Permitted

Canadian low-voltage SPDs are certified to the CSA C22.2 No. 269 series (269.1 for Type 1, 269.2 for Type 2, 269.3 for Type 3), and CEC Rule 26-420 covers the installation and connection of low-voltage surge protective devices, with additional guidance in Appendix B.

Cascaded protection

No single device does the job. Industrial practice is a coordinated cascade: a high-energy Type 1 or Type 2 at the service takes the bulk of the strike energy, a Type 2 at each distribution board reduces the residual, and Type 3 devices at sensitive loads clamp what is left plus any locally generated transient. The impedance of the conductors between stages is part of the design — it is what lets the upstream device see the surge first.

The ratings you actually select on

Lead length: the installation detail that decides performance

An SPD's published VPR is measured with essentially zero lead length. In the field, the conductors from the breaker to the SPD and from the SPD to ground have inductance, and during the microsecond rise of a surge, that inductance develops a voltage of its own that adds directly to the clamping voltage the equipment sees.

The practical rules are simple and they are worth real money:

• Keep the leads as short as physically possible. Mount the SPD on or immediately adjacent to the enclosure it protects.
• Keep the leads straight. No service loops, no coils, no 90-degree bends — bend radii are made generous and sweeping.
• Twist the line and ground conductors together to cancel the loop area.
• Never route SPD leads alongside the conductors they protect, or the surge will couple straight back in.

A perfectly selected SPD on 1.5 m of coiled lead can let through more voltage than a mediocre SPD mounted tight.

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4. Medium-Voltage Surge Arresters

Above 1 kV, the same metal-oxide technology is packaged as a surge arrester (IEEE C62.11), and arresters are graded by class:

Selection turns on two numbers working against each other:

• The arrester's MCOV must exceed the highest sustained line-to-ground voltage, including the temporary overvoltage the system produces on an ungrounded or resistance-grounded phase during a ground fault. On a high-resistance-grounded 4160 V system, an unfaulted phase rises to full line-to-line voltage, so arresters must be rated for it.
• The arrester's protective level must sit safely below the equipment's basic impulse level (BIL) — the gap between them is the protective margin, conventionally a minimum of 20%.

Mounting matters as much as selection. Arresters are installed as close to the protected equipment as the physical layout allows, with the shortest possible ground lead bonded to the same ground as the equipment frame. An arrester grounded to a separate electrode elevates the equipment frame relative to the arrester ground during a strike and does more harm than good.

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5. Undervoltage and Overvoltage Protection (RSOS B-8.05 / B-8.06)

Voltage faults that persist for seconds or minutes are a different problem from transients, and they are the ones that kill motors.

ANSI/IEEE device functions used for voltage protection

Why an overload relay is not undervoltage protection

A thermal or electronic overload relay measures current, and it is calibrated against a thermal model of the motor at balanced voltage. Two voltage faults defeat it:

• A voltage sag makes a loaded motor draw more current, which the overload will eventually see — but only after the motor has spent seconds at high slip and low cooling.
• Single-phasing on a delta-connected motor produces line currents that are uneven and not all elevated. The overload may sit under pickup on the very phase it is watching while the winding cooks. This is why a phase-failure relay or a modern motor protection relay with negative-sequence (device 46) capability is the correct protection, not the overload alone.

Voltage unbalance: the number and the consequence

Percent voltage unbalance is calculated from line-to-line readings:

$$\%\,\text{Unbalance} = \frac{\text{Maximum deviation from average line voltage}}{\text{Average line voltage}} \times 100$$

Worked example. A 600 V motor bus measures 598 V, 590 V and 612 V.

• Average $= (598 + 590 + 612) \div 3 = 600\text{ V}$
• Deviations: 2 V, 10 V, 12 V → maximum deviation $= 12\text{ V}$
• $%,\text{Unbalance} = (12 \div 600) \times 100 = \mathbf{2.0%}$

Two consequences follow, and both are steep:

1. Current unbalance is magnified. A small voltage unbalance produces a negative-sequence voltage that sees a very low impedance in the motor (roughly the locked-rotor impedance), so current unbalance typically runs 6 to 10 times the voltage unbalance — 2% voltage unbalance can show up as 12% to 20% current unbalance.
2. Heating rises with the square. Temperature rise increases by approximately $2 \times (%,\text{unbalance})^2$, so 3% unbalance adds roughly 18% to winding temperature rise — enough to halve insulation life.

NEMA MG-1 derating factors for operating a motor on an unbalanced supply:

Common industrial causes of unbalance

• Single-phase loads (lighting, welders, heaters) unevenly distributed across the three phases;
• A failed capacitor cell in one phase of a power-factor bank;
• High-resistance connections — a loose lug, a corroded fuse clip, a pitted contactor tip — which is why unbalance and infrared thermography investigations go together;
• Utility-side transformer tap or regulator problems;
• An open delta transformer bank feeding unbalanced single-phase load.

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6. Ground Fault and Arc Fault Protective Devices in This Family

The RSOS groups ground fault, arc fault and surge protection into the same sub-task because they are all non-overcurrent protective devices installed on the same distribution equipment. Their detailed operation is treated in the ground-fault section of this guide; the summary distinction is worth memorizing: