A properly designed PSU has foldback current limiting.  If properly sized, then a short circuit causes excessive current.  So the PSU cuts back power without flames.

However, many computer assemblers grossly oversize their PSU using a myth that more power makes a better PSU.  So that PSU turns a short circuit into flames; does not perform foldback current limiting.

Is your computer so hot as to also toast bread?  If not, then a 700 or 1000 watt PSU is excessive and unnecessary.  If a short circuit occurs, this grossly oversized PSU turns that short circuit hot as a bread toaster's toating wire.

ATX specs are quite clear.  All outputs from a PSU can be shorted together.  EVen that does not damage a PSU.  ATX specs even say how thick that shorting wire must be - so that every computer assembler will do it to verify a good PSU.

But if wires inside a computer are not big enough for an oversized PSU, then fire can result.

Molex connectors: each contact is speced to support up to 6 amps.  An assembler must add specification numbers to confirm that connector will not be overloaded.  A molex connector can be overloaded and still not cause melting.  Melting occurs when the connector is so grossly overloaded that the assembler should have know better just from basic knowledge. Never ignore numbers.