After a long response, some useful numbers. 12 volts at 120 mA is a
significant difference from, for example, a telephone wire. Telephone
wire can have 100 volts. If just a consumer product operating at 12
volts, then a regulator or current limited transistor switch may be
more than sufficient - depending in maximum source current and how much
PC board damage is acceptable. Some use Polyswitch from Raychem (now
Tyco) in series for backup protection; as noted earlier.

Polyswitch, for example, is often unacceptable for phone line (low
power) applications because of 60+ volts. Phone lines meet the
criteria in an earlier post that did not provide numbers - which is why
hedging on a Polyswitch recommendation was necessary.

What are failure criteria? If 12 volts rises to say 16, will that
cause a component to short circuit, then resulting in a short circuit
and (maybe) fire? Some 12 volt loads can withstand 30+ volts for short
periods. Others cannot. If used in automotive functions, then most
regulators will not meet the load dump criteria. Criteria typically
not found in computer 12 volt applications.

120 mA normal load implies a single point failure could consume
significant power (amperes?). In which case a second device (ie
Polyswitch) would provide good backup protection. This same protection
is use on computer keyboard and mouse ports. At low voltages and
currents, fuses have not been a preferred solution for maybe 20 years
now.

Nothing above could be recommended with an earlier post that provided
no numbers. Even 12 volts verses 60 volts would change the
recommendation. Again, replies will only be as good as the numbers
provided.

Derek Potter wrote:
...
FWIIW ,the small transistor circuit in question is a BS160 FET driving
a 12V load at 120mA but subject to possible short circuits as said.
The system fuse is 1A but fuses do not blow instantly so, with the
fairly high "on" resistance of the FET (rising as it heats up) there
is the distinct possibility of the TO92 device dissipating many watts
before failing. The electronic protection comprises a foldback
regulator and is perfectly adequate unless, of course it fails first,
leaving the circuit unprotected without any indication of the latent
problem.