power trouble with door-holder circuit

The installation has a Silent Knight 5820XL and several 5895XL power supplies; a surge went through the building and knocked out one of the 5895s, which we replaced. Everything works well except the door holders on circuit 6 - with everthing disconnected the panel terminals show 26vdc; upon adding the door-holders back in one at a time from the closest location we see a 1.5-2.0 voltage drop on the circuit that accumulates with each additional one. Everthing worked correctly prior to the surge, and there’s no as-builts or docs other than what I’ve been able to compile since taking over monitoring/service. Any thoughts on what could be causing this voltage-drop?

Sounds like one or multiple door holders were damaged in the surge. Disconnect the circuit, get a meter to it. You may need to replace some door holders.