The portion of the power system that is least prone to faults is Alternators. Let's understand why.

What is an Alternator?
An alternator is a machine that converts mechanical energy into electrical energy in the form of alternating current (AC). It is a type of generator that produces AC power.

Why Alternators are least prone to faults?
There are several reasons why alternators are least prone to faults:

1. Simple design: Alternators have a simple design with fewer parts, which makes them less prone to faults.

2. Robust construction: Alternators are built using high-quality materials and are designed to withstand harsh environments, making them more reliable.

3. Maintenance: Alternators require minimal maintenance, making them more reliable and reducing the chances of faults.

4. Protection: Alternators are equipped with protection devices such as overcurrent and overvoltage protection, which prevent damage to the machine and reduce the chances of faults.

5. Testing: Alternators are tested rigorously before they are put into service, which ensures that they are functioning properly and reduces the chances of faults.

Conclusion:
In conclusion, Alternators are the least prone to faults because of their simple design, robust construction, minimal maintenance, protection devices, and rigorous testing.