Pax*****,

Sounds so familiar.
But what they try to do is intimidating you with a lawyer. Most are scared to dead if they receive a letter from the court or lawyer.

As you probably knew, I'm a judicial expert for the Belgium courts, so I handle very often this kind of
problems.

Think Aiyub gave a very complete and correct answer. They only can blame you if you made professional faults, all the rest is inherent to the age of the used items.

Second, in the EU is a law that says that you're obligated to repair all the damage occurred due to a fault but that you also have to pay the damage that followed due to your fault. So, if you mis-wired a compressor, then you have to replace it for free but they can sue you also for the loss of goods due to this failure.
Loss of goods is normally covered by the insurance of the owner if he has an insurance for this. Most customers I know haven't a insurance for this and many times they ask me if I can 'arrange something' via my liability insurance.
Which I always refuse of course.
If you're sure you haven't made professional mistakes, let them go to court. Negative side of all this is that you will also need a lawyer which will cost you a lot.
We're insured for the cost of a lawyer.
Cost us +/- 700 € to 800 €/year which is very cheap.

You have to answer their letter within a certain time legally, otherwise you agree completely with what they're saying.

State in this letter also that you disagree completely their statements and point them to the fact that you have a written prove that you have warned your client.

If we can speak here of a client.

End your letter with a judicial phrase - i know the Dutch words for it and don't know the exact English translation and don't know also if this has the same judicial grounds as here - which says..will try to translate it...
'This letter was written under all reservation, with retaining of all my rights and without any disadvantageous declaration behalf my side'

Sometimes you write things that were meant completely otherwise then you wrote or that can be interpreted another way you meant, or they can interpreted it in a way that they attack you with what you wrote.

it's easier to explain technical matters