Published in IEEMA Journal, January 2008
In the first part of the article "Lightning & Surges"- Myths and facts, we had discussed about Disturbance variables - Immunity to Interference, Disturbance variables in Low Voltage networks, What is Surge and how they are caused, Common Myths and facts.
In this article, we will discuss in detail about various types of applications and the need for proper selection and recommended installation practices. The proverb "necessity is the mother of invention" comes in handy once again here. Nowadays, irrespective of income levels, buying power of Indians are increasing day by day. It is evident from the different new models of cars what we see every day and also the comfort zone gadgets- television, fridge, washing machine, home theatre, cooking ovens etc, etc. When it comes to entertainment and instant connectivity, the most important gadgets are Television and mobile phones. Hence these electronic gadgets have become necessity and hence, no longer luxury. Once we are used to these gadgets, we get addicted habitually and the failure or nonavailability of these gadgets irritates us to a great extent. Hence, having bought and used to it, it becomes our responsibility to make it available for use throughout the day so that we get uninterrupted usage.
Referring to my earlier article, once again the emphasis is on SPDs- Surge Protection Devices, as they are the only source of protection against transient (temporary) errors. We need to understand a very important factor now. The lightning arrestor (or air terminal) what we see on tall buildings, the associated down conductors and good earthing protects ONLY the building against direct lightning strikes. For example, when a tall structure is struck by lightning, the building may catch fire or collapse. The extent of damage depends on intensity of the lightning strike. Now, if the same building would have been protected by external lightning arrestor, the building is ‘well protected" from direct lightning strike. The important point which should be borne in mind is that, this external lightning arrestor system does not protect the electrical or electronic gadgets which are housed inside the building. SPDs are the only one to help in this case. Hence the concept is very clear- external lightning arrestor protects the building and SPDs- installed inside and near the electrical and electronic equipments protects them. But the catch here is, if a building is provided with external lightning arrestor, it is a must to have higher rating SPDs at the main distribution board level (a few kilo-amperes of lightning current handling capability which is based on 10/350 micro-second duration).
Hence, SPD should have minimum basic characteristics depending upon the application. For example, main DB SPDs contain Spark Gaps (as they are the only one which can withstand lightning current). The sub-DB SPD should contain large block MOV- Metal Oxide Varistor for Line- to-Neutral and Spark Gap for Neutral to Earth. The need for spark gap from Neutral to Earth is supported by the fact that, when GPR- Ground Potential Rise occurs, the entire current has to pass through this Neutral to Earth arrestor. Hence, its lightning current handling capability should be much higher. The other reason is, if MOV is used for Neutral to Earth, it not only have to have very big size but also suffers from leakage current. It means that, if ELCB- Earth Leakage Circuit Breaker is employed down the line, it will cause nuisance tripping. Equipment level SPDs employ MOV-Metal Oxide Varistor IGDT- Gas Discharge Tube ISAD- Silicon Avalanche Diode or Surge Diode. Each finds its application depending upon the Course or Precision Protection, response time, surge current handling capacity, voltage protection level (or let through voltage or limiting voltage) etc. In a typical example, a PLC- Programmable logic Controller which communicates to the host via R.F antenna will have MOV based SPD for power line protection, GDT based SPD for R.F (co-axial cable) protection and SAD based SPD for RS232/485 protection of 9 pin/l5 pid/25 pin D- type connector.
SPDs distinguish itself from the normally popular MCB / Opto-couplers / isolation transformers / stabilizers etc. due to one very important reason. They act in micro-second duration when compared to its counter parts which are primarily used to take care of steady state errors. Hence, these short duration transients have their associated current/voltage/ frequency in mega levels. Something like mega volts, kilo amperes and kilo hertz of frequency.
Normally, at the time of construction of new buildings and equipments, we get satisfied if we are able to achieve a earth resistance of approximately 10 ohms as recommended in national and international standards. But this resistance value is applicable for D.C or power frequency of 50 Hz. When lightning or distant lightning (which causes damage in 95% of the cases when compared to direct lightning) occurs, it usually has frequency to the level of few Kilo hertz. Hence, it is the impedance (predominantly Inductive reactance and to certain extent Capacitive reactance and the resistance) which creates the havoc. This can be best explained by a simple example. When lightning or distant lightning occurs, it has a typical frequency of 25 KHz and when it is applied in XL = 2?fL, the inductive reactance goes to a very high level because it is directly proportional to the frequency. This is the reason why, from lightning/ surge point of view, cables have to be used only of required length (may be with some tolerance for extra length). But certainly not very long length than the required length because, when we have extra length, we usually coil it with the result that the inductance is increased considerably. Hence coiling the signal cable or earth cable (normally done for aesthetic look) have to be avoided. The need of the hour is, trouble free equipments and not the aesthetic look of failed equipments.Different types of applications:
As the scope of this article is restricted to protection of low voltage equipments, the discussion will not be extended to protection of HT- High Tension or High Voltage equipments as it is beyond our scope.
If we go through the Installation Manual or User manual of House hold electronic gadgets, it explains one thing in common. "When you are not using for long time or going out on vacation etc, PHYSICALLY DISCONNECT the power cable and signal cable- Cable TV co-axial line, telephone line- to avoid failure due to lightning. But, in reality, how many of us read or practice this in this fast PLUG and PLAY world?
The different applications can be classified in to the following major categories:
Obviously, the selection of the right product for the right application becomes very important. Proper selection is the first step because there are more than 700 models/types of SPDs to select from. Here only the technical knowledge and experience of consultants and manufacturers comes handy.
Also, when it comes to data lines protection, the SPD should have both common mode and differential mode protection elements. Any signal line to ground is called common mode protection. Differential mode protection is between two signal lines and has no reference to ground.
So, to employ fool-proof protection techniques, the new projects or existing plants have to be surveyed thoroughly by the experienced lightning protection consultants. Only this will ensure proper selection.
Another aspect of selection is based on Zonal Concepts. Explained as LPZ- Lightning Protection Zones - in the widely popular and accepted IEC standards. In January 2006, IEC has published the following standard which act as the guide for Consultants, Contractors, Manufacturers and users.
IEC 62305 - Protection Against Lightning.
Above IEC covers, sources/risks/damages/losses and protection techniques in detail.Recommended installation techniques as per national and international standards
Proper selection is work half done, unless, it is supported by proper installation. So the manufacturer of SPD should act as a solution provider rather than a catalogue seller. Hence, the manufacturer- in the first step- doing site survey, helps in identification of equipments that need to be protected depending upon where they are physically located, identifying the correct product should be asked to supervise the installation because the best SPD in the world will fail to protect the equipments if not installed properly. Any new concept has to undergo this phase till it becomes more familiar. For example, no customer asks the MCB manufacturer to install the MCBs as it is in use for many years and well understood by the users. This highlights the fact that the SPD manufacturer should act as a solution provider rather than just seller. For any new or major projects, SPDs are asked to install by sub-contractors/ workers who have no or very little knowledge about SPD and its need.
Often many Engineers have one general question. If only one SPD is installed at the input of main incoming power, can the complete plant be protected? Others feel that installing SPD at the main power input of DCS is enough. Again IEC guides in this. There are 2 aspects discussed, One is Economic Consideration and the other is Technical Consideration. Under Economic consideration, it is explained that, instead of "no protection", employing SPD at the main input power side will avoid conducted surges through power lines. Under technical consideration, it is recommended to install SPDs for each equipment and for each incoming line. It is commonly found that IDU (In Door Units) and ODU (Out Door Units) used for R F application have only R F signal protection because it comes with the equipment manufactures (it is applicable only in the cases of major and reputed manufacturers). Inspite of this IDU and ODU fail because, surge entered through the power line and damaged the equipments. So, for complete protection, all the incoming lines to the equipment need to be protected.
There are only 2 principles/rules to be followed in installation of SPDs.
Rule 1 : Install the SPD as close as possible to the equipments that need to be protected.
Rule 2: The earth terminal of SPD has to be connected to panel/case earthing (which is finally connected to the plant neutral star point earth) as short as possible.
The above rules are based on the simple fundamental that- current takes the least resistance path - as SPDs are installed for the purpose of handling these currents, our job is achieved when these complete current is made to flow through the SPD rather than a portion of it going through "the equipment that needs to be protected" and damaging it.Checking the healthiness of the SPD
As they are passive devices, SPDs need not have to be checked for its healthiness everyday.
As SPDs are passive devices (does not need any power for its operation) they fail only due to 2 reasons, if earth wire is not connected at all and if the incoming surge is higher than the rating of the SPD. There are failures reported that in some cases - both SPD and the connected equipment failed. In other cases, SPD has not failed but the connected equipment has failed. Third case is SPD alone has failed. By proper selection and installation, all the above failures can be avoided. Present day SPDs are of self-resetting type. Once surge passes through it, it resets automatically and does not require manual intervention.
Simple thumb rule is, to check them after severe lightning is reported. Most of the power line SPDs which are connected parallel to the load comes with inbuilt failure indication. This can be local mechanical flag colour change and remote indication facility. Spark gap type SPDs can only be tested with the help of special testers which measure the insulation level to find out its healthiness.
Data line SPDs are mostly series connected type and hence its failure is self- revealing. Loss of signal at the contol room end indicated that something has failed. In this case, a simple check can ensure whether SPD has failed or not. If the loop works after bypassing the SPD, it reveals that SPD has failed. The manufacturer will confirm the same.Conclusion
Selection of Zonal level SPDs (main distribution board, sub-distribution board and equipment level) based on physical location and proper installation will ensure that equipments live upto the MTBF- Mean Time Between Failure- levels as published by equipment manufacturers. Finally, protection has to be 100%, i.e all the incoming lines have to be protected. For example, if a P.C has to be protected fully, then its Power line, LAN and RS232 port- all have to be protected.
Published in IEEMA Journal, January 2008