Reading the Manual for ENIAC, the World’s First Electronic Computer
It seems like the machine was temperamental. For example, it warns that the DC power should never be turned on without first turning the operation switch to “continuous.”
“Failure to follow this rule causes certain DC fuses to blow, -240 and -415 in particular.”
But the consequences are even worse if you opened the DC fuse cabinet when the D.C. power was turned on. “This not only exposes a person to voltage differences of around 1,500 volts but the person may be burned by flying pieces of molten fuse wire” (if one of the fuse cases suddenly blew). In fact, the ENIAC was actually designed with a door switch shunt that prevented it from operating if one of its panel doors was open, “since removing the doors exposes dangerous voltage.” But this feature could be bypassed by holding the door switch shunt in its closed position.