How is a washer circuit electrically affiliated with the main circuit breaker?

If the washer circuit is brand new and the main circuit breaker is about 40 years old, Is there good chance of a fire starting in our home?

How is a washer circuit electrically affiliated with the main circuit breaker?

If the washer circuit is brand new and the main circuit breaker is about 40 years old, Is there good chance of a fire starting in our home?

If the wire size used for the washer circuit is the proper size for that circuit and the circuit breaker is the proper size for the wire then all should be well. The main circuit breaker size is based on the amp capacity of the main panel and the wire size feeding the main electric panel from the meter base/ utility connection. A 40 year old main circuit breaker has no bearing on weather or not a fire will start as long as the breaker is in good serviceable condition and the entire system is installed to the National Electric Code or other accepted electric code and was inspected upon instillation for compliance with that code.

Agree with hvac. The new line should be protected by a new breaker. The main breaker most likely has a capacity 3-4 times the circuit side .

We cannot know what was done without seeing the breaker panel. What was supposed to be done and what was actually done can be two different things. The screws on the breakers and on the receptacles loosen over time and have to be tightened or there will be a fire as in your case. It's almost always caused by loose screws. You don't find out until the house burns down unless you go into each box and tighten up the screws. It would be a good idea to do it throughout your house. I bet you will find quite a few "surprises" and close calls, I did recently and had to replace several outlets.