Hi Steve:
First, one of my clients has been told by their 3rd party NRTL that a ground
bond test is required as part of factory/production routine tests, even though
it is not required by 61010-1. I have never run into this requirement in my
work history and I would be interested in hearing if any of you have.
In my experience, which dates back to UL1244 and UL478, no standard specified
routine production tests. Nevertheless, our contract with UL and CSA required
hi-pot and grounding continuity tests. These production tests have been
implemented ever since then. Regardless of the standard, the NRTL can demand,
through its contract with the manufacturer, production-line tests.
Second, just to be sure my interpretation is correct, 61010-1, section 6.5.2.4,
Impedance of protective bonding of plug-connected equipment, in the fist
sentence, it has a limit of 100 mOhms impedance and my read is that this is for
equipment with a power cord that has a plug on one end and a receptacle on the
other.
In my experience, the protective bonding impedance measured from any grounded
part (whether accessible or not) to the main protective bonding terminal (where
the power cord terminates on the equipment end) is 0.1 ohm maximum. The power
cord protective conductor resistance is not included in the equipment bonding
impedance measurement. (8 feet of #18 stranded copper wire is 0.051 ohms,
half of the 0.1 ohm requirement.)
(Back in the “old days” for me, the bonding impedance, including the power
cord, was 0.5 ohm.)
In the May, 2014, Symposium, I presented “Equipotentiality and Grounding,
Derivation of grounding resistance for equipment.” If you don’t have a copy, I
will be happy to forward a copy of either the presentation or the paper or
both.
(As an aside, only the fault current path need meet the impedance criterion.
Those grounded parts that are not in the fault-current path need not be tested.
And, those grounded parts that are separated from mains by double or
reinforced insulation are not required to have 0.1-ohm impedance to the PE
terminal or subject to the routine test. However, such testing may be
expedient for a production-line test.)
(I had a ground impedance failure on a PCB by a NRTL. The test was not the
current path. We pointed this out; the test was repeated on the actual current
path and passed.)
Then, in the same section, still under the title of plug-connected equipment,
it says for equipment that has a non-detachable power cord the limit is 200
mOhms. My interpretation is that plug-connected equipment with a
non-detachable power cord is when the power cord is hard wired to the unit
under test, but still has a plug for mains connection.
The 0.2 ohm limit accounts for both the resistance of the power cord and the
impedance of the equipment. I agree with you that a non-detachable power cord
is hard-wired the equipment.
Stay safe, and best regards,
Rich
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