Surge Arrestor Rating

Now is the time to contact the maker of surge arresters and pose your question to them.

1. ABB Surge Arresters - High Voltage Products
2. Surge Arresters - Cooper Industries
3. One should not guess when working with high voltage..

Us is the system voltage. Uc Continuous voltage. The Class will be No2. Therefore you will be looking for a surge arrestor that suits Us or Uc of your system.

The challenge of selecting the arrestor voltage rating is in determining the maximum sustained line-to-ground voltage that can occur at a given system location, then choosing the closest rating that is not exceeded by it.

For insulation coordination, protective ratios are calculated at three separate points within the volt-time regions. These are: the switching surge withstand, the full wave withstand and the chopped wave withstand. The following protective ratios must be met or exceeded if satisfactory insulation coordination is to be achieved, according to the minimum recommendations given in ANSI C62.22. Check other standards or as advised, call ABB or Tyco or K'line. They can advise for your local area as they have the data, or should have!

It all depends upon how your system is earthed. The SA's voltage rating is selected based on (Highest System Voltage x Ground Fault Factor). For solid earthing the GFF is 0.8. For impedance earthing, etc. it could be as high as 1.732. That's why for the same 33kV system, different rated SAs are available.

Hi Sir, A surge arrestor is made up of semi-conductor material internally. In layman's terms a fast "high voltage switch". Thus it conducts excess voltage to earth according to the MCOV voltage rating but remains open to circuit when excess voltage is not present. They have internal MOV blocks that have this property which switch excess voltages but remain open circuit to nominal voltages. They start to conduct voltage at its MCOV values. So everything depends on what protective margin you wish to use. When choosing a surge arrestor you can use a 33kv, 36kv and the 42kv unit all on the same system rated at 33kv. The voltage it sees is phase-ground so the rated voltage of eg. 33kv/1.73 is actually = 19.07kv. When applying the surge arrestor this value (33kv/1.73) should be lower than the MCOV, to prevent the surge arrestor from continuously conducting. Thus the 19,07kv is lower than 26kv, so the 33kv rated surge will work because its MCOV of 26kv is higher than 19,07kv. Meaning the surge arrestor will only start to conduct at 26kv. The MCOV must always be higher than the phase-ground voltage. The difference is in the protective margin. But they will all work. The savings could be in the cost of the unit, lower rated voltages are more cost effective. Smaller margins may provide better protection. Another consideration could be to change the class (the Ka), the higher the fault level the longer the lifespan. Alternatively look at a surge arrestor in a "drop out" form such as the "Firewall" available from 3kv-36kv. This unit is replaceable from ground via a link stick. It has an insulated bracket that holds the surge arrestor in parallel. When the surge arrestor is spent, it disconnects itself from the network and is replaceable from ground. The once off investment of the insulator hardware makes maintenance and downtime a thing of the past and enables one to employ more surge arrestors on your network without the need for future, outages for surge arrestor maintenance. Live Line Technology has the drop out surge arrestor available in these voltages - http://www.liveline.co.za/ll-arrestor.php