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Old 3rd Jul 2011, 10:08 am   #8
neon indicator
Retired Dormant Member
 
Join Date: Jun 2010
Location: Co. Limerick, Ireland.
Posts: 1,183
Default Re: Is the mains 'earth' good enough?

Historically UK (inc N.I.) was fig1. Ireland for a long time has been fig2.

If you are Fig1. and the radio uses mains earth, then the "aerial" connections, especially the "Earth" are best isolated by 1000nF 1kV.

If you are Fig2. or Fig3, then an isolation capacitor may be not important in one sense. But if there is a wiring fault in the house, a capacitor stops your Radio Earth conducting fault currents.

If the Mains Earth is poor, then all the filters in all the SMPSUs etc may put current via your mains earth on radio and flow to "earth" via your RF earth. The current will be a mix of 50Hz and switcher noise. Ferrite clamp near box end on all interference producing equipment and ferrite clamp on plug end at Radio set hugely reduces HF and higher interference. For LW & MW multiple turns of the entire cable (interference producer and/or radio) on an old LOPT core is needed to get enough common mode inductance. The 1nF capacitor will mostly stop 50Hz earth currents.

If the equipment has the concentric box symbol and a two core flex, then don't fit a three core flex and don't connect chassis to earth.

If it's before the days of that symbol and test, then megger live and neutral to the chassis. If if passes and came with a 2 core flex, don't fit a 3 core. If it fails investigate why, if there is no fault, then connect a 3 core flex. If the leakage is too high at mains voltage (more than ??), then there is a fault and adding an earth will trip a correctly installed RCD.

I don't have the figure for leakage. If you touch an old metal case Sky box that is plugged in and not connected to anything you may feel leakage. It's at a permitted "safe" level. Someone may know what that is for UK.

Because neighbouring houses (or possibly apartments) are on different phases (380v to 440V between "lives") then any un-isolated inter-house wiring or shared aerial, satellite dish etc must meet EXTRA safety regulations of Earth bonding, how and where the earth bonding is done and minimum size of earth wires for bonding.

Don't share wiring, aerials or dishes without qualified advice. A computer RJ45 / Cat5e or ethernet cable can be shared (as these use built in isolation on network adaptors), but there is a right and wrong way of doing even that! Especially if there is a screen on Cat5e or BNC on coax ethernet can come in contact with a chassis.
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