Surge Suppression Devices

By Mike Holt – a short summary of the requirements contained in the Understanding the 2008 NEC, Volume 1 Textbook.

Avoid transient voltage problems by understanding how to correctly apply Article 285.

A voltage surge can destroy valuable equipment, or even shut down an entire facility. Providing protection against transient voltage spikes for expensive electronics and mission-critical equipment is vitally important.

Hard-wired surge arrestors installed on premises wiring systems over 1,000V (1 kV) are covered by Article 280 of the National Electrical Code (NEC). Article 285 covers surge protective devices (SPDs) for applications under 1 kV. SPDs may be either surge arrestors or Transient Voltage Surge Suppression (TVSS) devices.

Most Article 285 surge arrestors operate by “clipping off” the highest level of voltage and diverting it away from the load. In this manner, the voltage on the conductor is limited to a predetermined level.

The spark gap arrestor is a simple device and is an example of a surge arrestor. Though it’s an Article 280 device, describing its operation helps us understand the operation of the surge arrestors covered by Article 285.

One lead of the spark gap arrestor is on the protected conductor; the other is on the conductor of an alternate path. At the arrestor, what is commonly called an “air gap” separates these leads. In reality, this gap is filled with an inert gas rather than air. This gas normally provides such a high impedance that current flow is nearly zero. When a voltage spike occurs, the increased voltage finally reaches a high enough level to overcome the impedance of the air gap, and current is conducted through the gas and onto the alternate path. The gas continues to shunt current onto the alternate path until voltage drops to a level closer to normal.

Article 285 surge arrestors don’t use a spark gap with an inert gas and don’t operate on the short-circuit effect. Instead, a solid state device such as a Metal Oxide Varistor (MOV) or Silicon Avalanche Suppression Diode (SASD) “switches on” at a certain voltage. Unlike the spark gap arrestor, these devices conduct only the overvoltage portion of the transient or spike. The effect is called “voltage clamping.” And they must clamp within a limited range of voltages.

That’s where TVSS comes in. A basic TVSS cascades a series of MOVs to progressively reduce the spike until it’s no longer a threat. Thus, one device will handle the entire voltage range of the spike at a given location. But a TVSS may do more than that, because the diversion technique isn’t the only method of dealing with power spikes.

While it’s true that diversion devices tend to dissipate much of the transient energy as heat, they do not eliminate transient power spikes. They primarily reduce the voltage levels and incidentally reduce the energy levels. That is not always sufficient protection.

It may also be necessary to add filtering to the protection scheme. Filtering devices do not divert the transients, but use inductive and capacitive elements to absorb them. Filtering devices may be incorporated into a TVSS, and such devices are covered by Article 285.

Hardware

When planning an SPD installation, look at your hardware options. You must decide which performance specifications you need for each device at any given point of protection, such as at a feeder distribution panelboard as opposed to a main service disconnect. It may be a challenge to sort through all this and come up with a well-engineered solution, but it’s an even greater challenge to go back and do it over if you didn’t comply with these three SPD requirements at the outset:

• Where used, the SPD must be connected to each ungrounded conductor of the circuit [285.4].
• SPDs must be listed for the intended use [285.5]. UL 1449, Standard for SPDs clearly states that these units limit the maximum amplitude of transient voltage surges to specified values. They aren’t lightning arresters. Again, we’re back to the concept of a system, with layers of protection based on energy level.
• Each SPD must be marked with its short-circuit current rating (this requirement doesn’t apply to receptacles containing surge protection [285.6]). An SPD must not be installed where the available fault current exceeds that rating. SPDs of the series type are susceptible to failure at high fault currents, and if the AIC rating of an SPD is less than the available fault current there can be a hazard of explosive destruction of the SPD resulting in damage to equipment and possible injury to personnel.

Installation

The NEC does not require the installation of SPDs, since they are not an essential component needed to meet the minimum safety requirements of the Code [90.1(A)]. But if you do install SPDs, you must install them according to the requirements of Article 285.

Don’t install SPDs in any of the following applications [285.3]:

• Circuits that exceed 1 kV.
• Ungrounded systems, impedance grounded systems, or corner-grounded delta systems, unless listed specifically for use on these systems.
• Where the voltage rating of the SPD is less than the maximum continuous phase-to-ground voltage available at the point of connection.

Avoid unnecessary bends in SPD conductors, and don’t make the conductors any longer than necessary [285.12]. You must not connect more than one conductor to a terminal, unless the terminal is identified for multiple conductors [110.14(A)].

SPD ratings

SPDs are rated Type 1, Type 2, Type 3 or Type 4.The SPD Type is not a performance rating, but more of an energy rating. The NEC is concerned only with the safety aspects, so it provides limitations on where you can install an SPD based on its Type.

Type 1 SPD

You can install Type 1 SPDs [285.23] on the supply side of service equipment [230.82(4)]. It’s also common to install them on the load side of service equipment as allowed in Section 285.23(A)(2), based on the energy levels encountered there.

When installed at the service, the grounding conductor of a Type 1 SPD must be connected to one of the following:

(1)        Service grounded conductor.

(2)        Grounding electrode conductor.

(3)        Grounding electrode for the service.

(4)        Equipment grounding terminal in the service equipment.

Type 2 SPD

Type 2 SPDs [285.24] are allowed to be installed on the:

(A) Load side of the service disconnect overcurrent device. Type 2 SPDs are not listed to accommodate lightning-induced surges beyond their capacity, so don’t install them on the line side.

(B) Feeder-Supplied Structures. Type 2 SPDs are allowed on the load side of the building or structure overcurrent device.

(C) Separately Derived Systems. Type 2 SPDs are allowed on a separately derived system, where they must be connected on the load side of the first overcurrent device.

Type 3 SPD

Type 3 SPDs [285.25] are allowed to be installed on the load side of a branch-circuit overcurrent device up to the equipment served. But only if the connection is a minimum conductor distance of 30 ft from the service or separately derived system disconnect. Type 3 SPDs can be thought of as a point of utilization device.

Type 4 SPD

A Type 4 SPD is mentioned in NEC Article 100 as a component assembly, such as a relocatable tap, and is not usually part of a permanent installation covered by the Code, but should still be an important part of the full SPD protection plan.

Conductors

Line and grounding (bonding) conductors for SPDs must be at least 14 AWG copper or 12 AWG aluminum [285.26]. An SPD may be installed between any two conductors, including ungrounded conductor(s), grounded conductor, and grounding conductor. The grounding conductor and grounded conductor can be interconnected only by the operation of the SPD during a surge [285.27]. All grounding connections must conform to Article 250, except as noted in Article 285 [285.28].

Permanent transient protection

A good transient protection system that includes a properly-specified SPD scheme installed per Article 285 can easily pay for itself during one power event. The economic justification for installing such a system is compelling for almost any facility.

SPDs tend to lead fairly short lives compared to other electrical equipment because of the very nature of their operation. If you just install them and forget about them, you may later find that the SPDs have been damaged while performing their intended role and are no longer able to provide protection during a transient.

While you can easily tell when a transformer blows or a motor fails, an SPD failure doesn’t announce itself by providing dramatic fireworks or by shutting down production. An SPD performs its function under abnormal conditions and for short periods of time. While functioning to protect the electronic equipment, an SPD may be damaged so that it no longer provides surge protection, even though the load remains energized. For this reason, a failure may not be noticed when everything is going well unless you have SPD instrumentation to alert you to the failure.

The NEC doesn’t require preventive maintenance, monitoring, or testing of SPDs. Nor does it require that they be tested after a storm or after a known power event. From an operational viewpoint, however, these measures allow you to ensure that the SPDs are still functional and waiting to provide the protection that led to their installation in the first place.

 

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