Neutral/Earth Voltage


Thread Starter

Bryan Rooney


I'm having a problem with an analogue addressable fire panel in a new hotel. The system incorporates 4 fire panels each with 3 analogue loops. All of the some 12KM of fire alarm cabling is screened firetuff. When I connected an oscilloscope, at one of the panels I can see noise on the earth - traces of the analogue comms from the other loops and panels. This is causing interference as comms from one loop is being induced onto others via the earth screen.

A voltage of approx 1.2VAC exists between Neutral/Earth on the fire alarm supply. The earth loop impedance at this point is 0.8 Ohms. This is a separate supply from the main switch room. A socket close by the panel despite having a very different supply route has exactly the same N/E voltage but an earth loop impedance of 0.31Ohms. Supplying the fire panel via this serves little or no effect.

NOTE: This N/E voltage was 0.8 volts for months and has only recently increased to 1.2VAC without any apparent building changes.

The earthing is TNC-S Neutralised at supply entrance and back-up earth rod supplied

Any thoughts would be much appreciated.


bryanrooneygda at gmail. com