NotTaR of Television Sets : Shock and/or spark when connecting cable..  
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Shock and/or spark when connecting cable or other A/V components

TVs with hot chassis - where signal ground is actually line connected and at some intermediate (and dangerous) voltage - will have an isolation block in between the tuner and antenna/cable connections. TVs with isolated power supplies may have some bypass capacitors between the power supply and signal grounds (including the A/V shields if there are A/V connectors). It is possible for a failure to result in a serious safety hazard where the RF (antenna/cable) or A/V connectors become electrically live.

However, a tingle or small spark might be normal. RFI bypass caps between the AC input and shield on the connector could result in some leakage - 50 V or more might be indicated using a high impedance multimeter. This is harmless. Reversing the plug in the AC socket (if it is not polarized or if you are using an unpolarized extension cord) might eliminate or greatly reduce the effect.

Nonetheless, it should be checked out. Measure the resistance between each side of the AC plug and the RF and AV connector shields. It should be 1 M ohm or more. Test for voltage between the cable (or other device) connector and earth ground. If there is anything significant, test the resistance on the device between its shield and its power plug as above - other devices may have RFI bypass caps or be defective as well.