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Expansion Joints, Metal Raceways and PV


Q:

Hi Doc,
here in Indiana land we mostly use schedule 40 conduit and armored cable and flexi steel conduit I have not had the opportunity to couple EMT,and my question is on 300 7 (B) expansion fittings,are these special couplings ,can you explain more of this,or a picture paints a thousand words ,and you could add some words to explain more .
Thanks much really enjoying this stuff.

A:

Here is a report that is about a famous PV fire on a rooftop in Bakersfield. Nobody was hurt and the building didn't burn down, but the fire led to a big push towards ungrounded PV systems, which would not have caught on fire in this case.

Here is the Fire Report:

http://nfpa.typepad.com/files/target-fire-report-09apr29.pdf

The second to the last and the third to the last pages have images of expansion joints on EMT.

Here is a SolarPro article regarding the fire:

http://solarprofessional.com/articles/design-installation/the-bakersfield-fire-a-lesson-in-ground-fault-protection/page/0/4

With grounded PV systems, there are leakage currents that leak from the equipment through the ground fault detection and interruption fuse (GFDI fuse). If the system is big, the leakage currents can go undetected and in the case of the fire, there was a second ground fault and a loop of current that arced. I recommend reading the SolarPro article. We also discuss this in the video lectures.

To answer your question about conduit. When PV circuits are going through a house, they must be in a metal raceway or MC cable up until the first readily accessible dc disconnect. Some people think that this means ac inverter output circuits (I don't). Most people keep it simple and put their PV circuits in EMT.

You can see in the fire report and the SolarPro article, that there are some good examples of EMT melting, so think what would have happened with plastic. It is ok to use schedule 40 PVC when it is not inside of a building and is common in Hawaii where there is a lot of corrosion on steel EMT.

Aloha!

Sean White


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