Even though we all know how to use a search engine, I found this link relevant:
http://www.bchydro.com/business/investigate/investigate763.html

This pocketbook taught me alot about HID the first time I got it:
http://www.advancetransformer.com/uploads/resources/hid-troubleshooting-guide.pdf

Here is the link to the page you can find that pocketbook on. I didn't realize how advance expanded their available liturature. More Sunday afternoon reading:
http://www.advancetransformer.com/resources/literature.jsp