You're referring to why the jumper on a 3-phase disconnect used for single-phase 240V is wired the way it is, instead of just directly connecting the line side of A to the load side of A. Here's the breakdown of the reasoning

Why Not Just a Direct Jumper?
Overcurrent Protection

Simply jumpering from line to load would bypass the fuse/breaker protection on that phase. The purpose of the disconnect is to safely interrupt the circuit under fault conditions (overload, short circuit). With a direct jumper, a fault on the A-phase load would potentially draw excessive current through the other phase's protective device (fuse or breaker) instead of being isolated quickly and safely by its own device.

Equal Load Sharing (If Future 3-Phase)

While it's not the primary reason now for a single-phase setup, this jumper configuration is preferred when the disconnect might be repurposed for a three-phase load in the future. It ensures that if the other two phases are used, they'll have their own individual protection.

Why The Current Configuration Works



Using Both Poles

The common wiring configuration involves bringing one leg of the 240V supply to a line terminal (say A), and then using a jumper to connect its load terminal to one of the line terminals of the B phase. The other leg of the 240V supply is connected to the line terminal of the B phase. This effectively uses both poles of the disconnect, but feeds the single-phase load through both sets of contacts in series. This is a safer way to feed a single phase load.

Overcurrent Protection Retained

When there is a fault on the single-phase load, both the 'A' and 'B' phase overcurrent protective devices (fuses or breakers) will trip. The load sees the protection that is installed.

In Summary

The jumpering method used on a 3-phase disconnect wired for single-phase isn't the most intuitive, but it prioritizes safety by maintaining proper overcurrent protection for the load and allowing for easy future conversion to three-phase operation.

Disclaimer:Electrical work can be dangerous. If you're not a qualified electrician, please consult one for any wiring or electrical projects. Flag for review