Shielding high-speed cables from an EMP I used to work at a facility which dealt with a huge power deposition (2 terrawatts) from a high-current pulse. When we would string up detectors to measure events on the scale of a few nanoseconds, we would do it in the way illustrated in the figure bellow. What confused me is that the metal braid shielding around our coax signal cables was only grounded on one end (i.e. it was only connected to one of the faraday cages), and I never got a clear answer on why this was the case.
Why would this provide better shielding than connecting the braid to the Faraday cages on both ends?

 A: In your scenario it might be interesting to know which direction the pulse comes from, what was its polarisation, and whether that was always the same. This will indicate which way the prevailing charge motion in the cages would be.
Surface currents in screens/cages are localised, they are induced by local external EM fields. 
If the metal braid were connected presumably that would result in larger currents across it due both to different local conditions at opposite ends and also an infinite supply of charge as both ends would be earthed. 
There's another possibility depending upon how the detectors worked, because the signal you are measuring seems to be the same as the noise you are trying to screen out. Thus if your earthling is allowed to fully respond to the pulse it could make certain types of detectors less able to pick it up?
The answer is not straight forward as practical geometries can be complex. It wouldn't surprise me that some trial and error was involved in determining that the above configuration was usually better.
A: An EM pulse will typically induce a current in any conductor it meets.
If the braid is connected at both ends, current can flow freely and for an intense field it can be very large. Faraday cables and co-ax cable are not perfect at their jobs; hit them with enough raw power and enough of the pulse will get through to degrade the measurement signal.
By breaking the current path, the braid still provides E-field protection and the current in it will be greatly reduced.
This system of single-end shielding connection was common on old audio equipment and low-bandwidth phone exchanges.
