Again with respect the designers would be used to compensating to temps of -20°C. These things happen and if you can be better at the job then apply for it instead of complaining. We're a Nation of grumblers and don't want do anything about it but complain.
I thought they said it was due to freezing conditions outside the tunnel and the higher temperature inside the tunnel. ie. What was unprecedented was the weather conditions particularly in northern France with heavy snowfall and very, very cold temperatures outside of the tunnel.
The conditions caused condensation which brought about electrical problems in the tunnel.