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When using an ”electrical safety analyser” that displays mohm on the ground lead from plug to device chassis, (Fluke 180 or equiv), what is the underlying technical reason for measuring the ground lead resistance of the device under test (DUT)? Should one look for the point of lowest reading, or look for conductive surfaces that could be touched, and get their resistance measurements too? The maximum allowable is 0.1ohm. Why is that the cut-off value?

Hi Gary,

 

Thank you for your question regarding electrical safety checks of medical equipment, it is our pleasure to help.

Many facilities, especially the military and hospitals, have requirements to have all chorded devices inspected for electrical safety.  These checks often include small personal devices such as stereos and kitchen appliances, to larger devices such as test equipment.  The basic premise of these tests is to prevent accidental fires and short circuits in a facility due to someone plugging in a faulty device.  By using an ohm-meter and testing the resistances from the 120-volt plug, you can simply and quickly determine if the device is basically safe.  Here is what is typically checked:

  1. Hot to Ground measures an open (too resistive to measure)
  2. Neutral to ground measures an open
  3. Chassis to ground measures below 0.1 ohms (subtract the resistance of the test leads at temperature)
  4. Hot to Neutral is at least 50-ohms (greater is better) when switched “On”

The reason for these four (4) simple tests is as follows:

  1. The Hot wire and ground wire must not ever touch.  This is a short circuit.
  2. The Neutral wire must not have a path to ground except at the first service disconnect (the main electrical panel).  If the ground and neutral are touching, return currents will travel down the ground wire and onto the chassis of objects along the path of the circuit.
  3. The ground wire should be bonded to the chassis of the device (if metallic).  A low resistance confirms this bond.  There are some exceptions when dealing with what is called a “isolated ground” system which is designated by a triangle on the 120-volt outlet.  Isolated grounds are not actually “isolated”, they are simply bonded to ground at the electrical panel and not at the device.  If you are plugging into one an isolated ground outlet, you will really need to study up on the regulations regarding isolated grounding to understand how your test will be effected.
  4. The hot and neutral wires are providing the power to the device.  If the resistance is too low, a short is bound to occur.  Writing the measured resistance down and tracking the change over time is a common maintenance technique for predicting failures of equipment before they happen.  If your device when new measured 1,000-ohms, and has been dropping by 100-ohms every year (900 at year one, 800 at year two, etc.), you should not be surprised when your device fails at year ten.

Measuring the chassis ground does require you to find a solid electrical connection to the frame, paint and other plastic components can prevent accurate readings.

There are many different rules in regards to the allowable resistance between the hot and the neutral wires in 120-volt alternating-current (AC) systems; a minimum 50-ohms is common, but others may want 100-ohms.  Direct-current (DC) systems have different values which may be lower.   In particular, speakers for sound systems can get as low as 2-ohms.  You will have to be aware of what the regulations are for your individual clients, if they have such rules.

Now in regards to your specific medical equipment, the best thing you can do is to determine what the average range of resistances occur for each of your devices when new.  For example, if Device-A has an average resistance between the hot and neutral when turned “on” of 2,500-ohms, and Device-B is 5,000-ohms; you should expect your device to measure within some range of those averages when you do your test.  Readings below or above the expected range should be considered as a sign that something is wrong.

We hope you have found this information useful.  If you should have any further questions, please do not hesitate to call us at 310-318-7151.

Best regards,

The Engineering Team at E&S Grounding Solutions

 

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