Introduction:
The least expensive protection for over-current in a low voltage system is a rewirable fuse. A rewirable fuse, also known as a fuse wire or cartridge fuse, is a type of electrical protection device that is designed to break the circuit when the current flowing through it exceeds a certain level. This helps to protect the electrical components and prevent any damage or hazards.

Explanation:
Below are the reasons why a rewirable fuse is the least expensive protection for over-current in a low voltage system:

1. Cost:
A rewirable fuse is one of the most cost-effective options available for over-current protection. It is relatively inexpensive compared to other protection devices such as circuit breakers or isolators. This makes it a popular choice for low voltage systems where cost is a major concern.

2. Simple Design:
Rewirable fuses have a simple design consisting of a fuse wire or a fuse element connected between two terminals. When the current exceeds the rated value, the fuse wire melts and breaks the circuit, interrupting the flow of current. This simplicity in design contributes to the low cost of rewirable fuses.

3. Easy Replacement:
One of the advantages of rewirable fuses is that they can be easily replaced. When a fuse wire melts due to over-current, it can be easily rewired or replaced with a new one. This eliminates the need for additional expensive components or tools, making it a cost-effective solution for protecting low voltage systems.

4. Availability:
Rewirable fuses are widely available in the market and can be easily purchased from electrical stores. They come in various ratings and sizes to suit different applications. The availability of rewirable fuses adds to their cost-effectiveness as they are readily accessible and do not require specialized procurement processes.

5. Compatibility:
Another advantage of rewirable fuses is their compatibility with a wide range of low voltage systems. They can be used in various applications such as domestic electrical systems, small industrial installations, and automotive circuits. This versatility makes rewirable fuses a cost-effective choice for protecting low voltage systems.

Conclusion:
In conclusion, a rewirable fuse is the least expensive protection for over-current in a low voltage system due to its low cost, simple design, easy replacement, availability, and compatibility. It provides an effective and affordable solution to protect electrical components from damage caused by excessive current. However, it is important to note that rewirable fuses have certain limitations such as slower response time compared to circuit breakers and the need for manual replacement after each overload event. Therefore, the selection of an over-current protection device should consider the specific requirements and constraints of the low voltage system.