DC Protection required?

[Clive P]

Hi Everyone,

I have been a member of the forum now for a few months, but this is the first time I have been on the PV Forum, my apologies if this has been brought up before.
A friend of mine is connecting up some stand alone panels ( 14 in all , 7 Banks of 2 in series as per the manufacturers instructions ). The maximum output of each bank of panels is 42volts , which are feeding a 24V Battery bank. The battery bank supplies a Cathodic Protection unit ( this is for Gas Pipeline protection ). My question is , should the outputs from the Panels be protected in some way, ie DP DC MCB or similar; or because they are below 50 V then this is not a requirement? I believe he is looking primarily at short circuit protection or is this limited by the panels themselves? There is no connection to any Mains voltage , but he has put down an Earth rod and this has a reading of 900 Ohms ( extremely bad shale type ground ).
The Batteries themselves will be protected with a 30A HRC fuse.

Any thoughts would be much appreciated.

 

[Worcester]

The panels limt themselves on short cirvuit protection, in fact one of the standard tests that we do as part of an installtion is to short circuit them and measure the current. - they are a 'current limiting device'

Where large numbers of of strings (a string = 1 or more panels conenctied in series) are conencted together and could theoretically power another string if it was faulty, then each string has a dc fuse inserted.

Check out this guide : http://www.bre.co.uk/filelibrary/pdf/rpts/Guide_to_the_installation_of_PV_systems_2nd_Edition.pdf - OK it relates to buildings, however it is a good start. (para 2.1.10 specifically)

Because also it is a gas pipeline I am sure that there will also be specific regulations covering cathodic protection - check out those - or point us to them and we may be able to interpret them for you in this application.

 

[Clive P]

Thanks Worcester, just the type of answer I was looking for, and I will follow up on the link.

 

[moggy1968]

The fuses operate at only slightly above short circuit current level so in the event of a short probably wouldn't blow anyway, so are rather worthless.

 

[Worcester]

@moggy1968 The problem comes when you get a number of strings attempting to power a single faulty string. Think of it like a shorted battery in a bank of parrellel connected batteries, the shorted one 'sucks' the power from the others and could explode. Not as extreme with panels, however if you get one faulty string, electricity takes the easiest route, as I understand it, so the other strings connected in parallel could now well revese feed the faulty string, in the case of five strings in parallel, with up to 4 times the normal string current. - thats why the fuses are rated only slightly above the short circuit current - it protects the 'faulty' string from even greater damage.