I re read the initial post, and it occured to me that it appears that external equipment is being blamed for the failure, this is in its self a failure to understand what a designer has to consider when considering EMI.

It is a requirement (well it is in Europe at least), that equipmet shall be able to withstand the effects of external influences from other equipment, and shall not itself present any untoward influence upon any external equipment, be it by conducted or radiated emissions.

As has been stated there is really only on way to get this right, and that is proper shielding, proper power supply design and filtering, filtering of all incoming and outgoing signals, pcb with proper power supply planes, proper ground planes, correct device decoupling, and contary to popular belief, don't take what the device manufacturers say regarding decouping as gospel, they are very often WRONG, a common mistake is to have multiple capacitors of differing values, don't do it, stick to a single value, 100nF is a good start.

We design equipment that has to be able to cope with quite harsh transients on a -48V supply (Telecomms) upto 1000v and often a few hundred amps flowing as a fuse ruptures. It can be done, and quite easily with proper design.