But if arson was the case, how can police determine this then?
Someone doing this intentionally would be very unlikely. Damaging the service in some way would likely only result in localized damage to the house because of the way the service is installed. Services are installed so to minimize the possibility of fire damage.
As was said, if the whole house was involved, the cause has to be a primary to secondary cross. That would put high voltage (7,000 to 12,000 volts) on the service going into the house which would put it on all the house wiring which would cause fires to erupt all through the house. All electrical equipment would burst into flames.
Neither of those videos above illustrate this. Those are only downed high voltage power lines.
The only place a primary to secondary cross could occur is out on the street when a high voltage conductor at the top of the poles is damaged for some reason, falls down and contacts the secondary conductors below. There is no way somebody is going to do that intentionally.
We have a video of a whole neighborhood involved but it was a long time ago and I can't find it.