How an electrical fire starts

As I have mentioned in previous articles I have been working in group homes that where poorly maintained and improperly inspected.
and we had one hell of a scare the day the 6 boys ages 12-16 moved in.

Because the home does not have a central AC system in it they went out and bought individual AC units for each bedroom bad idea as some receptacle wiring is new and some is old and about an hour after the units where started myself and a helper decided to go and cool off in a side sitting room and we noticed a smell of burnt rubber not a good thing. Possibly a bad new AC unit no they all checked OK we then got out a simple heat gun and found that a double switch plate was warm in the sitting room by 20 degrees more than rest of the wall. But How ? both switches where off.
Well it ends up after carefully opening the switch box we found behind it 2 old splices going thru the box feeding other areas in the house which where soldered together and then taped with rubber tape then cloth friction tape. Because the splice was loose this allowed a high resistance connection to occur which caused the heating of the wires Wait a minute Soldered wire splice in a house ? Yes in fact it was a popular way to do it before wire nuts became available and it is still legal to do but where possible mechanical connectors are preferred way to do it.
So what was causing the splice to heat up ? after it was opened it was found that the soldering job was not properly done and it probably would have stayed that way for ever and not been found till the heavy air-conditioning load was put on the home.SO could there be more bad splices and could a fire result? Most definitely any time you subject old wiring to a new heavier load a tragedy is always possible.Is there any way to find and prevent this? Yes number 1# replace all the outlets and switches and make sure all connections are tight to specification 2# there are specialized analyzers which can be plugged into a receptacle that subjects it to several different load tests and will tell you if the receptacle is safe. 3# You could also install the New Arc fault Circuit interrupter breakers which could also help prevent problems in old wiring as well.
Luckily in this case we found it and no one got hurt but had we not been there very well there could have been a fire and some one could have lost there life. Tonight thou some one was watching over them.