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.
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.
- 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.
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 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.
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.
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.
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.
Achieving Graded Surge Protection
From 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.
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.
Resource: Grounding, bonding, shielding and surge protection – G. Vijayaraghavan
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