The Grounding Continuity Test (aka Earthing Continuity Test) is one of the most common product safety tests. It is performed on products that have a ground (earth) connection by Certification labs as part of the product Certification and by electrical product manufacturers on 100% of production. Let’s review the elements of this test.
a) Grounding vs. Earthing: Grounding & Earthing are interchangeable terms with identical meaning. In the US & Canada, the term “grounding” is used, while most other countries use the term “earthing”. Accordingly, UL & CSA standards use the term “grounding” while IEC & EN standards use the term “earthing”. This whitepaper uses the term “grounding” which can be replaced throughout with the term “earthing”.
b) Protective Current Rating: Some standards specify using a test current level that is based on the “protective current rating”. The protective current rating is the current rating for the device that provides “branch-circuit overcurrent protection”. It is the rating of a circuit breaker or cartridge fuse = this device can be part of the product, but it is usually part of the building electrical system (i.e. the circuit breaker in the breaker panel).
c) Protective Earth Terminal (P.E.): The P.E. terminal is the connection point for the mains grounding connection and is one of the two connection points during testing. For a cord connected product, the P.E. is the point where the ground connection from the mains power terminates at the product enclosure.
Purpose of the Test:
Test Objectives: There are two objectives to keep in mind:
1) Not Powered: The product is not connected to power during this test.
2) Two Different Methods: The Ground Continuity test has two different test methods – check your standard to determine which applies to your product.
3) Low Current Test Method: The low current version of the test is used by many UL standards for product certification. It is also the basis for most production line ground continuity tests. For the low current version of the test, any reliable means to verify ground continuity is all that is required. The continuity setting on handheld voltmeters will suffice, as will any other commercial continuity checker.
4) High Current Test Method: The high current test requires a Ground Continuity tester or other means to generate the specified test current. The test voltage is adjusted to achieve the desired test current, and then ohms law is used to calculate the test impedance. The test current and the test time can vary and are each discussed in detail below.
5) Test Limit: For the high current test, the maximum impedance of the grounding system is 0.1 ohms. A few standards allow for higher limits, but this is usually as an offset for power cord impedance for products with a permanently connected power cord. In addition, standards that anticipate test currents over 25A sometimes specify the limit as a maximum voltage drop across the P.E. grounding system of 2.5V instead of the 0.1 ohm limit (note that a 2.5V drop @ 25A equals 0.1 ohms).
6) Test Current: For the high current test, the test current is usually 2X the protective current rating. An AC or DC test voltage is permitted.
7) Test Time: The test time can vary from 2 minutes to as high as 10 minutes. The higher the protective current rating, the longer the test time. Consult your standard for the test time vs. protective current rating table.
8) Test Locations: The ground impedance limit applies between the P.E. terminal and the following test points. For convenience, the test can be performed from the grounding pin of the plug to the following test points:
9) Avoid Contact Resistance: Make sure you are using a suitable probe tip for contact with the surface being tested = a clamp works much better than a pointed probe tip to avoid contact resistance (although at times it may be difficult to find a place that a clamp can grip).
10) Test Leads: Be sure to use test leads and connectors suitable for the test current. Note that the lead impedance is not part of the grounding resistance limit – some ground continuity testers will allows you to “zero out” the test lead resistance thereby excluding it from the displayed result. A “zero” feature can also be used to remove the power cord impedance from the measurement when testing cord connected products.
11) P.E. Trace: If your P.E. system includes a trace on a printed circuit board, the trace must also comply with the Limited Short-Circuit test of CSA C22.2 No. 0.4.
Test Exceptions: See your standard to determine if any of these exceptions apply:
Production Testing: Many standards require the Grounding Continuity Test be performed on 100% of production. Some UL standards only require the production test on products with a grounding type plug, specifying a test between the grounding pin of the plug and accessible dead metal parts. All other aspects of the test methods specified above apply. Production testing is not required by some standards on products that are field wired (conduit connected) and provided only with terminals or leads for electrical connection.
As you can see, we don’t simply perform the tests because they are in the standard. Each test in
the standard has a set of objectives that relate to the 6 Hazards of Product Safety. The Ground
Continuity Test is performed as part of the Risk of Shock compliance review. Verifying reliability
of the product’s grounding system is crucial to insuring it continues to provide protection from a
Risk of Shock during a product fault condition, a potentially serious hazard that could lead to death
by electrocution. It is therefore a very important test – another test that directly saves lives. This
is why the Production Ground Continuity Test is required on 100% of certified products that have
a power cord with a grounding type plug.
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