I was fiddling about setting up a new car and reversed the polarity to my esc , I was only using one battery from a saddle pack to check the esc when I Connected it to the battery the Bec started smoking instantly and I disconnected instantly, the esc is a a HW xerun v3.1 there was no motor connected at the time and my receiver was not plugged in either. After all this I checked and esc still switches on and fan runs ok nothing was hot straight after it happened (esc or battery) but there is a slight burn mark on the Bec board just near the wires is it fried regardless that it switches on or is it possible that it will be ok

The only way you will know for sure is to test it with motor and Rx. You are unlikely to damage the Rx and Motor by testing further.

BEC's are fragile things and IMO cause many so called ESC failures. If the ESC runs a motor then you have dodged a bullet but I am no hopeful, sorry

It's probably at least been damaged. Reversed polarity is something that should really best be avoided - for a reason.

It is probably the additional cap board that went "poof". There is a reverse biased diode on there that is designed to blow with the idea that it protects the ESC. Remove the cap board, inspect it, and replace it. The ESC will still run without it, but obviously it's there for a reason (many theories on this!!), so replace it.

The ESC should still be fine

This is really what I was hoping to hear (fingers crossed) it's still ok cause I really don't want to literally blow another couple hundred on this build. I have had a look at the external cap board and seems to be a little tarnished just where the wires meet it and this is what started smoking not the esc itself. I have emailed Angelo from cobraracing and hopefully he will give me the same advise as this
Thanks
Edit: turns out you were correct brew99, my esc is SAVED thanks to the cap board.

I've had an ESC which I plugged in reverse polarity (not intentional) and it took out 3 fets but the others were fine. Given it was a 120A Esc (R10 Pro) and I was only running 21.5 blinky, it was fine to continue with albeit a little smelly.

That is correct, reverse polling an ESC will cause damage to the 4x FETS per phase polarity, (6x polarity phases = 24 FETS) if your ESC still works after reverse polling it chances are you have killed a few FTES and are not aware of it. These FETS don't stand much of a chance when powered in reverse with such high burst currents in the batteries these days.

What are the fets?

MOSFETS, FETS for short, they are used to switch or amplify electronic signals

I think if you haven't reversed polaritied a speedy you aren't doing it right. I did it one night at 12th scale with a brand new $249 Muchmore Fleta 1S. Smoked like a chimney, all the lads around the table looked at it and said welcome to the club.

So depending on the amount of damadge to the fets determines wether it will work on a certain turn of motor ?so in nexus' case would that esc no longer have worked for say a mod motor or it's possible it would have?

It was just such a simple mistake but man was I really kicking myself after I did it and was thinking how stupid it was that it happened

Each FET has a pulsed current rating at a certain temperature and also a continuous drain current rating.

Say each FET has a continuous drain current rating of 40A, 40x4 = 160A so the manufactures would rate the ESC at around 160A or less to allow for temperature loss. 160A is more than enough for a modified motor, now half the FETS from 8 per phase to 4 per phase (2 FETS per polarity phase ie 2 FETS for the positive A phase and 2 FETS for the negative phase) then your continuous drain current rating of 160A will change to 80A. This in turn determines the motor you wish to use with the ESC.

In your situation I would give the ESC an easy life with a high turn motor like 21.5t and cut your losses. You are very lucky it even still works.

Spot on, from my mucking about with an Eagletree logger, a 21.5 motor is not going to pull any more than 40A from a dead start (surge rating) and at full throttle down a long straight it was around 12-13 A

Ive decided I'm going to get a new esc regardless but -I'm not asking cause I want to do it but cause I'm curious-if it so happened that some of the fets were damadged and you were trying to run a high turn motor what would the outcome be? Would it not run the motor to its full potential or would it cause other types of issues?