Lightning Arrestor for 11kV Construction Power Supply System

I am afraid you would be compromising on your protection if you go for 15kv arrestors. for 11kv system with solidly earthed neutral only 9 kv arrestors must be used. may be you can go for 9kv 10ka arrestors.

11KV Lightning Arrester

11KV/1.732= 6.35 KV

7KV or 9 KV Lightning Arresters can be used

Pl. do not give incorrect advices, if you are not sure. LA rated voltage is NOT calculated by simply dividing the line voltage divided by root3. LA rated voltage is selected by: highest system voltage x co-efficient of earthing. For 11kV, the highest system voltage is 12kV and for solid earthing, the co-efficient of earthing is 0.8. So, for 11kVsolidly earthed system, the rated voltage of LA should be 12 x 0.8 = 9.6kV. The nearest standard LA available is 9.0kV.

I've never had to spec lightning arrestors on our existing lines so i cannot comment on the voltage rating issue directly, as i am NO expert. But ponder a question or two: 1. Do you know why they are failing? That may give you a few ideas. Over here, the correct term ofnre device is actually a surge arrestor, but it is also commonly referred to as a lightning arrestor. Are you getting switching surges for the supplier or back from the installation? A datalogger should help you here. Or are you really absorbing actual lightning surges? 2. Also with the logger, check how good the supply voltage is. Sometimes they can go outside the 10% allowance for extended periods and i reckon the LAs would not like that! 3. My gut feel would also be to rather go for 10 kA versions rather than going for a higher voltage, depending on the answers to 1 and 2 previously. Just going for higher voltage ratings without understanding the cause could result in more expensive equipment such as transformers failing. (which i guess was the reason for your query in the first place!)

When considering what size lightning arrestor to install, one must consider two parameters.

Arrestor rating and MCOV(maximum continuous operating voltage)

For each arrester rating, there is a recommended limit to the magnitude of voltage that may be applied continuously

The arrestor rating must be selected such that the maximum continuous power system voltage applied to the arrester is less than, or equal to, the arrestor's MCOV rating.

Arrestor rating and MCOV are not the same. A 9kV rated arrestor has an MCOV of 7.65kV (http://www.geindustrial.com/Newsletter/lightning_arresters_guide.pdf) No surprise that they are failing.

To put it another way. Your over pressure blow off valve has a lower set point than your operating pressure.

You (or your supplier/contractor) need to select an arrestor with an MCOV greater than 11kV. (these are all RMS voltages)

Select with care and note all other selection criteria before proceeding with final selection.

Thanks Mr.Wal for your reply and the valuable information regarding the website. However, I am afraid,you have overlooked my original post wherein, it is clearly mentioned that our 11 kV system Neutral is solidly grounded. Hence,your following statement is not correct :

Arrestor rating and MCOV are not the same. A 9kV rated arrestor has an MCOV of 7.65kV (http://www.geindustrial.com/Newsletter/lightning_arresters_guide.pdf) No surprise that they are failing.

Those figures are from the source.

The phase to neutral voltage will be less than the 11kV phase to phase voltage, that is true. The lower rated LAs should be able to withstand that ...but....

You haven't provided the LA arrangement. With full protection you would have LAs phase to phase as well as phase to ground and neutral to ground. Each LA location has different MCOV.

Which ones are dieing? Has your supplier come up with any answers?

In our case all the LAs are phase to ground and pole mounted.

No, the supplier has not yet come up with any answer.

Will it be OK if we go for 9kV,10 kA discharge current LA ?

Same shunt threshold rating (9kV) with higher current handling (10kA v 5kA) is ok if the cost and size is not an issue and the grounding is good for 10kA.

Not sure how that will help you. LAs have a limited life dependent on the number of activations. Are they failing because they are doing their job frequently or some other issue?

Have you performed any event logging?

Rattle the supplier's cage really hard!!!!