Have been testing a factory, 2 DB fed from a TPN supply via an isolator up till today all circuit on the 2 boards have been OK. But today lost the Zs on the Grey phase on both boards but still had 230/400 volts,Zs OK on Brown and Black. DB boards mounted on a freezer type of wall with a door beside them, the door slammed shut, Grey Zs back. Thinking bad connection I have tried all the connections and they are all tight. Now Zs on the Grey phase is less than Brown and Black, I have worked out the readings and the Grey phase is about correct and the Brown and Black are on the high side Grey Zs 0.02 ohms Brown and Black 0.13 ohms hope this makes sense CJS ?:|