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Earth Bond Tester

Currently available from my eBay account - Link to Items for Sale

This is a essentially a Milliohm meter, comprised of a low voltage current regulated (1 or 10A) source and a millivolt meter, allowing you to measure a low resistance presented between the two probes. A Kelvin connection is used to remove any influence of the test leads, with a separate signal and power connection the both the heavy duty brass probe and the Crocodile clip. The unit also has a 13A socket on the top, connected to mains power but with an interrupted earth connection, which runs via a True RMS 1MHz filtered current measuring circuit. This is for measuring earth leakage currents generated by a device, an important electrical safety factor. 

  • 1A measuring range allows 0.2-4 Ohm measurements to be taken.
  • 10A measuring range allows 0 - 0.35 Ohm measurements to be taken.
  • Earth leakage can resolve up to 7.5mA. 

The open circuit voltage across probes is approx 12v DC (smoothed). 

Most safety guidelines specify a earth leakage of less than 3.5mA or 1mA depending on application.  This device could be used for PAT testing, alongside a Megger for insulation / flash testing if required. They are useful for equipment builders to verify that each part of a machine or appliance has an appropriate earth path. They can also be used to test earth bonding on vehicles, main engine earthing etc, since the measurement is taken between two leads - you can place the clip on the engine lifting stud and probe the car chassis earth points. 

They are also superb for checking Motor windings / armatures, home water pipe earthing arrangements, any low impedance paths (PCB traces, heating elements etc). 

The units were originally constructed by the Medical Physics department of East Kent Hospital for the NHS, mainly from RS Components parts. The display is a LASCAR "off the shelf" LED panel meter, the Toroidal transformer an off the shelf part (with twin windings - the measuring circuit and current delivery are kept floating to maintain the kelvin measuring point at the test points.)

The current delivery is presented for around 5 seconds when the push button on the probe is pressed, to minimise the risk of damage and to ensure that the probe is in place before current delivery (to reduce pitting). The manual, which is attached to this page as a .pdf, includes a block diagram which is repeated below.

They are housed in a sturdy aluminium equipment case which is then stored in an RS Components padded zip case, which has space for the main unit and a front pocket that can house the test leads and a side pocket that can house the power lead. Since these leads are not disconnectable this prevents you from zipping the case all the way up...

Since the units are used and from a working environment testing X-Ray machines and imaging equipment, they are in various conditions. The main units, being of sturdy construction, are generally in good cosmetic condition, the cases are usually missing a zip or two and usually the carry strap due to a broken quick release clasp.

The units were regularly calibrated by Trescal and are usually between 2 and 4 years out of calibration.

Calibration can be adjusted by altering the current delivered by the regulating circuit (a PCB mounted potentiometer allows for this) and/or trimming the trim pot on the panel meter for nulling. I have often found the 10A mode needs re-trimming as the wire/wound potentiometer for trimming the current limiting shunt suffers from surface corrosion - a simple exercise back and forth and a re-trim to 10A using an avo / any DC ammeter good for 10A and it is spot on again.

An internally mounted 0.2 Ohm 1% resistor allows for basic testing prior to each use.


Internal Photo
The 10A w/w trim pot is clearly visible.

Alex Grant,
Feb 20, 2016, 4:28 AM