Leads and fuses: the cheap parts that carry the rating

Leads: the rating is on the lead, not in the listing

Test leads carry their own measurement category and voltage, printed on the lead body. Fitting unrated leads to a CAT III meter gives you a CAT III meter with unrated leads — the combination is rated at the lower of the two.

Almost no lead listing publishes this. Read what is printed on them when they arrive, and follow the instructions supplied with your instrument.

Leads: what actually fails

The insulation, at the strain relief where the cable leaves the plug. That is where flexing concentrates, and it is also exactly where your hand is.

Inspect the whole length before every use on a live circuit. Leads are the cheapest part of the kit and the part that fails first. Silicone insulation stays flexible where PVC hardens and cracks, which is why professional leads feel soft.

Fuses: four specifications, all of them binding

The manual gives four figures for the current-input fuse:

• dimensions (5 × 20 mm (0.20 × 0.79 in), 6.3 × 32 mm (0.25 × 1.26 in))
• current (500 mA, 10 A)
• voltage (250 V, 600 V, 690 V, 1000 V)
• characteristic (F, FF fast; T time-delay)

All four have to match. Not the fuse that fits the holder, not the one with the right current, not the glass one from the drawer.

Why the voltage rating is so high on instrument fuses

Not because of the circuit the fuse normally sits in. Because of what it must interrupt when things go wrong: a meter connected across a supply with the dial on amps is a near short circuit, and clearing that fault means breaking an arc without it re-striking.

A 250 V fuse cannot reliably break an arc at 400 V, and the fault continues through the component that was supposed to stop it. That is why instrument fuses exist as a separate product at a separate price.

And the mistake the fuse is protecting you from

Leads left in the current sockets while measuring voltage. Put them back in the voltage sockets every time the meter goes down.