Can safety chains be used at the top of a fixed ladder?

No. Safety chains are not allowed as fall protection at fixed ladder entrances. OSHA specifically addresses safety chains in regard to fixed ladder openings in the preamble to the walking-working surfaces final rule by stating:

“... OSHA believes that chains are less protective than self-closing gates. Self-closing gates and offsets are passive fall protection methods that automatically restore guardrail protection as soon as the worker passes through the opening or offset area. Neither method requires the worker to take any action to restore that protection. However, if employers provide double chains at entrances to ladderway floor or platform holes, their employees would have to remove the chains and reattach them once they pass through the opening. If workers forget or fail to reattach the chains, they and others in the area could fall through the hole. Workers also are at increased risk of falling through the hole once they enter the area inside the guardrails to climb down the ladder because they have to turn around and away from the hole to reattach the chains and risk falling backward into the hole.”