Electronic – How to check earthing using handheld digital multimeter


I would like to know to how to check for earthing (electrical) ina house using handheld DMM.

Best Answer

The simple, but somewhat unreliable method uses a long wire and a DMM. Connect one end of the wire to a known, good earth contact (maybe next to the location where your fuse box is installed). Measure the resistance from the other end of the wire to the ground connector of the outlet/device to be tested. If you read a small resistance, you are probably good: Your multimeter tests the connection from your device's ground connection through the house installation to the central ground and your long test wire. However, there is a gotcha: Maybe somewhere along the installation, there is a bad contact that will fuse even with a small (say less than 1 A) current. Once it fuses, you end up with a dangerous situation, because your real fuse will not have blown at just 1 A and still supply current to the system. Your DMM will use just some micro- or milliamps and hence will not be able to "blow" the unintentional fuse (read: bad connection).

The solution to this problem would be a current source (strong bench supply, professional tester - but the latter es expensive) limited to whatever maximum current your installation and fuses are rated at (16 A, 20 A, maybe even 30 A?). Feed the current through the PE connection from the wall outlet to the central ground connection of your house and measure the voltage drop using the voltage range of your DMM. You're good when the voltage drop or the resistance (voltage divided by current) is small. Local regulations will tell you how small is small enough. To get an idea, you might try to calculate what resistance you expect using the cross-section and length of the wiring you have used in your installation.