No no no no. No. No no no.
Look at the writeup again for the exact words used and if they are the same words used by one of the standard recognized authorities like NFPA 70e, some type of already established and followed work rule. If you were written up for one of those, yes your job / employment would be targeted.
I have never heard of "securing the circuit" as the commonly used phase, or seen it in reference material. My guess is that the writeup is not referencing an already established rule, and if this is a fact that is true, the supervisor saved your job,.by writing you up for some nonsense phrase, and not something else that does have an already established work rule, must follow requirement (like no LOTO, working wirenuts with the power on ... there are so many).
He saved your job by referencing some not established phrase, if it says "securing". Like Infinity has been saying, "secured the circuit" what does that mean? you should be happy about that or at least recognize the signifcance. You may have been written up for something that is not a mandated rule.
But of course no one will ever forget and trying to make it an HR case, when you visit the zoo, you are advised to not poke sticks into the bear's cage. Iguanas and reptiles you can poke at, they are slower than you and used to it. But not with mammals, especially the meateaters that outweigh you by a lot.
If there were some way to make it go away quietly, it would have about the same odds as Powerball. Stepping on the dog poo and then poking sticks into the bear's cage, you are the hazard.