CRT tubes usually have stern safety labels warning to not exceed specified voltage, not because there will be an electric breakdown, but because this will gener

CRT tubes usually have stern safety labels warning to not exceed specified voltage, not because there will be an electric breakdown, but because this will generate vastly more X-rays. So the quotation "glass formulation limits the voltage" is probably correct, but it is supposed to mean: "glass formulation determines how opaque to X-rays the tube is, and what acceleration voltage can be safely used while remaining in compliance with the health regulations". I think this is what it is supposed to be. Lead based glass is so common in CRTs precisely because of this reason.