Fluke 6500: My Extension Leads Are Very Long & Fail Earth Continuity Tests
his is because the extra length is increasing the earth resistance to a point where it exceeds the 0.1 Ohm limit. Table 15.1 on page 74 of the IEE Code of Practice 3rd edition gives the limit for IEC and extension leads as 0.1 Ohms plus the resistance of the cable. You can work this out using the table VII.1 on page 124 of the C of P where it lists cable sizes and the resistance per metre and as long as the tester reading is within the calculated limit, you can pass the test. There are testers on the market which have a built-in ‘Limit Calc’ where you can enter the size of the cable in mm and the length in metres and the tester will calculate the correct pass/fail limit.
The 6500 has this feature. Once the lead has failed on the continuity test and ‘LEAD FAIL’ is displayed together with a cross, press the ‘SETUP’ key to recalculate the limit allowing for the length; the ‘LIMIT CALC’ screen will be displayed and the ‘LENGTH’ field will be in reverse video. Use the up and down arrows to increment or decrement the length displayed to the measured length. Pressing the right arrow key will move the cursor to the ‘SECTION mm2’ field where you use the up and down arrows to increment or decrement the size of the cable. Press the ‘YES’ key to confirm the calculation. The tester will re-calculate the pass/fail limit and the test will pass.