Hey Squiggles, the earthing of telecommunications cable screens at both ends is not used on a regular basis. It is mainly used in AC rail traction environments and is very expensive to implement. The cables have aluminium sheaths and the sheaths are earthed at 1km intervals. I was involved extensively in the testing of the effectiveness of this screening and I can assure you that it works well.
I would not even consider using it in an environment like a sub station where there are significant earth currents running through the structure. You would be running part of the disturbing current through the shield and as you found out the effect on the interference would be to increase it.
However, in an EV the traction battery is generally only earthed to the chassis at one point and there should therefore be minimal earth paths for the traction current.
The design of electrical noise reduction in systems has always been a hotly debated topic and due to the complexity of the issue it is impossible to come up with a one approach fits all scenario. Let's just say that if you have an interference problem you should firstly shield the cable and earth it at one end only. If the problem still exists try earthing the shield at both ends, but only if there are no earth return currents going through the chassis.
BTW I agree completely about the state of the telecommunications system in Australia. I think you will find the majority of the issues arise from poor maintenance by our favourite Telecomms company. TXXSTRA
