220V Insulation monitor, Low resistance/Fault from non earth TVs

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1moley
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220V Insulation monitor, Low resistance/Fault from non earth TVs

Postby 1moley » Sun Dec 16, 2018 11:31 am

Hi Lads,

We have a three phase 220V supply in accommodation.

Our insulation monitors are our showing a fault. I have sourced this to various televisions in cabins but these televisions have no earth cable in power lead.
Just a plastic prong on uk plug. Sometimes the earth presents itself straight away or after alittle time. I spoke to shore electricians and they said this is a common problem. Only way to fix is to install a isolating transformer in each cabin.

My limited knowledge leads me to believe insulation monitors, monitor the capacitance leakage between earth and phase. And if these TVs are producing high frequency resonance this could cause the insulation monitor to believe there is a partial fault.

Any explanation and further fix would be most appreciative.
Many Thanks in advance

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Big Pete
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Re: 220V Insulation monitor, Low resistance/Fault from non earth TVs

Postby Big Pete » Sun Dec 30, 2018 2:50 am

I have had this experience several times and I have heard possible explanation, firstly you must remember that TV sets are designed for use ashore where the power supply consists of one live wire and a Neutral wire which is connected to Earth at the Star point of the supply Transformer. On a ship we have two live wires, and no "Neutral".
The chassis of the TV can be used as an "Earth return" the same as with a car, cheaper than having a 2 wire plus Earth system,(so longer as the chassis is doubly insulated) and another point is that the aerial amplifier system is Earthed.
If the screening cable of the aerial wire is Earthed to the "Neutral" (actually live Phase) chassis in the TV and to a real, Ship's Earth in the aerial amplifier, we have an obvious problem! On a ship, live power from the chassis will be fed into the screening cable and be connected to the real Earth at the amplifier creating a direct path from the "Neutral" to Earth. This is also potentially hazardous.
With signal cabling the screen is only supposed to be Earthed at one end, to prevent current flowing through the screen and inducing a current in the signal wire, which will cause interference to the signal.

If the fault is in the power supply a 1:1 isolating transformer in the power supply will resolve it, if it is in the aerial system an indirect coupler can be fitted in the lead which isolates the Earth and transmits the signal by Capacitive or Inductive coupling, or alternatively the Screening on the aerial wires can be stripped back so that they do not connect to the TV sets.

I am sure that a little experimentation will produce a SAFE fix.

Happy New Year!

BP
It is always better to ask a stupid question than to do a stupid thing.

1moley
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Joined: Fri Dec 07, 2018 10:56 pm
Currently located: North Sea

Re: 220V Insulation monitor, Low resistance/Fault from non earth TVs

Postby 1moley » Wed Jan 02, 2019 11:31 pm

Hi Big Pete, Happy new year to you to!

Interesting read. Got some boys coming down next crew-change to install isolating transformer in all cabins.

The idea about the signal cable screening being connected to the earth/neutral is a good idea. We also have a fault with our vsat that provides the tv satellite signal so maybe this is connected?

I shall check and report my findings.

Many thanks
Moley


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