Surge Protection of Electronic Equipment-Transient Voltage Surge Suppressors

Surge Protection of Electronic Equipment

 

 

Introduction

Generally, power circuits have components that have large thermal capacities, which make it impossible for them to attain very high temperatures quickly except during very large or long disturbances. This requires correspondingly large surge energies. Also, the materials that constitute the insulation of these components can operate at temperatures as high as 200 ºC at least for short periods.
Electronic circuits, on the other hand, use components that operate at very small voltage and power levels. Even small magnitude surge currents or transient voltages are enough to cause high temperatures and voltage breakdowns.

Transient Voltage Surge Suppressors

Transient Voltage Surge Suppressor (TVSS) is a device that every data center or mission critical facility should have.
Why should every data center have one and what does it do you ask?
The purpose of a TVSS is to eliminate or reduce damage to data processing equipment and other critical equipment by limiting transient surge voltages and currents (surges) on electrical circuits.
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These transients or surges may come from inside a facility or may be injected into a facility from outside.

What is a transient?

A transient surge is a short blast or pulse of high energy that can either come in its natural form such as lightning or produced by other equipment.

Transients caused by other equipment are usually caused by the discharge of stored energy in inductive components. Some examples are electrical motors, such as those used in elevators heating, air conditioning, refrigeration or other inductive loads. Two other sources are arc welders and furnace igniters. These transients are capable of causing significant damage to equipment and electronics.
The transient causes damage to a device when the transient voltage exceeds the weakest exposed component’s ability to withstand that voltage. Transients normally flow into equipment via electrical conductors, but other paths are common. These paths include: telephone lines, data-com line, measurement and control lines, DC power buses and neutral and ground lines.
To protect against these surges designers recommend the installation of a TVSS devices that connects to all points of potential voltage threat and limit this voltage to a level below the equipment “withstand” voltage. The TVSS device absorbs or diverts all the energy present in the surge and clamping or holding the “let through” over voltage down to a level safe for exposed circuitry.
TVSS protection is typically applied at several points throughout of facility. These locations include the service entrance point, distribution panels, branch panels and the individual circuit.
As you can see a TVSS device is important to mission critical electrical system and its benefits are great. A TVSS is a low cost protection device that will help to reduce downtime or production losses. It helps to extend lighting lamp and ballast life expectancy. The TVSS will help in reducing motor stress and overheating and is a constant protection of data processing and digital equipment.
If you mission critical facility does not already have TVSS devices installed we highly recommend it. If you are not sure if your system has them installed we suggest asking your engineer or electrician to verify. It is a small price for additional peace of min.

Main causes of transient over voltages

1. Lightning Strike

a) A lightning Strike can have a destructive or disturbing effect on electrical installations situated up to several miles away from the actual point of the strike.

b) During a storm, underground cables can transmit the effect of a lightning strike to electrical equipment installed inside buildings.

c) A lightning protection device (such as a lightning rod or a Faraday cage) installed on a building to protect it against the risk of a direct strike (fire) can increase the risk of damage to electrical equipment connected to the main supply near or inside the building.

Left: Direct lightning strike on overhead line; Right: Indirect lightning strike on ground

Lightning strike on lightning rod

The lightning protection device diverts the high strike current to earth, considerably raising the potential of the ground close to the building on which it is installed.
This causes overvoltages on the electrical equipment directly via the earth terminals and induced via the underground supply cables.

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2. Switching operation on the power distribution system

The switching of transformers, motors or inductances in general, sudden variation of load, disconnection of circuit breaker or cut outs lead to over voltages that penetrate the user’s building.
Significantly, the closer the building is to a generating station or substation, the higher the over voltages may be.

Medium voltage disturbance transmitted to low voltage side of transformer

It is also necessary to take into account mutual induction effects between the high voltage power line and aerial sections of the low voltages lines as well as direct contact between lines of different voltages caused by accidental breaking of cables.

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3. Parasitic interferences

These are freak interferences with indifferent amplitudes and frequencies that are re-injected into the electrical supply by the user himself or his environment.

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4. Disturbances generated by the user

Disturbance generated by the user

These interferences have little energy but their short duration, their steep wave front and their peak value can have harmful effects on the proper functioning of the sensitive equipment causing either disruption or complete destruction.

This is so because of the very small electrical clearances that are involved in PCBs and ICs (often in microns) and the very poor temperature withstanding ability of many semiconducting materials, which form the core of these components.

 

As such, a higher degree of surge protection is called for if these devices have to operate safely in the normal electrical system environment.
Thus comes the concept of surge protection zones (SPZs).
According to this concept, an entire facility can be divided into zones, each with a higher level of protection and nested within one another.

As we move up the SPZ scale, the surges become smaller in magnitude, and protection better.

• Zone 0: This is the uncontrolled zone of the external world with surge protection adequate for high-voltage power transmission and main distribution equipment.
• Zone 1: Controlled environment that adequately protects the electrical equipment found in a normal building distribution system.
• Zone 2: This zone has protection catering to electronic equipment of the more rugged variety (power electronic equipment or control devices of discrete type).
• Zone 3: This zone houses the most sensitive electronic equipment, and protection of highest possible order isprovided (includes computer CPUs, distributed control systems, devices with ICs, etc.).

The SPZ principle is illustrated in Figure 1.

Figure 1 - Zoned protection approach

We call this the zoned protection approach and we see these various zones with the appropriate reduction in the order of magnitude of the surge current, as we go down further and further into the zones, into the facility itself.
Notice that in the uncontrolled environment outside of our building, we would consider the amplitude of say, 1000 A.

As we move into the first level of controlled environment, called zone 1, we would get a reduction by a factor of 10 to possibly 100 A of surge capability. As we move into a more specific location, zone 2, perhaps a computer room or a room where various sensitive hardware exist, we find another reduction by a factor of 10.

Finally, within the equipment itself, we may find another reduction by a factor of 10, the effect of this surge being basically one ampere at the device itself. The IEEE C62.41 indicates a similar but slightly differing approach to protection zones.

The idea of the zone protection approach is to utilize the inductive capacity of the facility, namely the wiring, to help attenuate the surge current magnitude, as we go further and further away from the service entrance to the facility.

 

The transition between zones 0 and 1 is further elaborated in Figure 2. Here we have a detailed picture of the entrance into the building where the telecommunications, data communications and the power supply wires all enter from the outside to the first protected zone.

Notice that the surge protection device (SPD) is basically stripping any transient phenomena on any of these metallic wires, referencing all of this to the common service entrance earth even as it is attached to the metallic water piping system.

Figure 2 - The transition from zone 0 to zone 1

Similarly, the protection for zone 2 at the transition point from zone 1 is shown in Figure 3.
Here as we address the discrete level between the first level of controlled zone 1 and perhaps the plug-in device taking it into the zone 2 location, we can see surge protection devices are available that handle the telecommunications, data and different types of physical plug connections for each, including both the RJ type of telephone plug as well as coaxial wiring.

Figure 3 - The transition from zone 1 to zone 2

This is a common design error where there are two points of entry and therefore two earthing points are established for the AC power and telecommunication circuits.

The use of the TVSS devices at each point is highly beneficial in controlling the line-to-line and line-to-earth surge conditions at each point of entry, but the arrangement cannot perform this task between points of entry.

 

This is of paramount importance since the victim equipment is connected between the two points. Hence, a common-mode surge current will be driven through the victim equipment between the two circuits despite the presence of the much-needed TVSS.
The minimal result of the above is corruption of the data and maximally, there may be fire and shock hazard involved at the equipment.

No matter what kind of TVSS is used in the above arrangement nor how many and what kind of additional individual, dedicated earthing wires, etc. are used, the stated problem will remain much as discussed above. Wires all possess self-inductance and because of −e = L dI/dT conditions cannot equalize potential across themselves under normal impulse /surge conditions.

Such wires may self-resonate in quarter-waves and odd-multiples thereof, and this is also harmful.This also applies to metal pipes, steel beams, etc.
Earthing to these nearby items may be needed to avoid lightning side-flash, however.

 rom the above, it will be clear that the type of surge protection depends on the type of zone and the equipment to be protected. We will further illustrate this by example, as we proceed from the uncontrolled area of zone 0.
Let us begin by talking about what happens when a lightning strike hits an overhead distribution line.

Here in Figure 4, we see the picture of the thunderstorm cloud discharging onto the distribution line and the points ofapplication of a lightning arrestor by the power company at points #1 and #2. We notice that the operating voltage here is 11 000 volts on the primary line and the transformer has a secondary voltage of 380/400 V typically serving the consumer.

We need to understand what is known as traveling wave phenomena. When the lightning strike hits the power line, the powerline’s inherent construction makes it capable to withstand as much as 95 000 V for its insulation system.

Figure 4 - Protections in zone 0

We call this the basic impulse level (BIL).

Most of the 11 000-V construction equipment would have a BIL rating of 95 kV. This says to us that the wire insulation, the cross-arms and all of the other parts, which are nearby to the current-carrying conductors, are able to withstand this high voltage.

Traveling waves and sparks over the lightning arrestor applied on a 11 000-V line might have a spark-over characteristic of approximately 22 000 V. This high level of spark-over protection is to enable the lightning arrestor to wait until the peak of the 11 000-V operating wave shape is exceeded before discharging the energy into the earth.
The peak of the 11 000-V RMS wave would be somewhere in the neighborhood of 15 000 V. As the voltage comes to the 22 000-V level and then stays there as the lightning arrestor performs its discharge, that voltage waveform travels on the power line moving very fast to all points of the line. At places where there is discontinuity to the electric line, such as points #3 or #4 in our chart, the traveling wave will go in at 22 000 V and then will double and start back down the line at 44 000 V.

This type of phenomenon is known as reflection of the traveling wave and it occurs at open parts of the circuit or even the primary of transformers. When the primary of our distribution transformer serving the building achieves 44 000 V, the secondary supplying the building is going to have an over-voltage condition on it.

Thus, points #5 and #6 on our chart require us to think in terms of some type of lightning-protective devices at the secondary of the transformer, the service entrance to the building and then further on into the building such as point #6 for the sensitive equipment to be fully protected in this facility.