Re: Charger Grounding (was Re: Charger Grounding Russco)

["Mander, Art" <amander@tristategt.org>]

There's plenty of notes on this subject in the archives, but it's real
important so it's worth reviewing.  Per 1999 NEC article 625-22 the charging
source for an EV should have a listed "Personnel Protection System" to
prevent electrical shock.  "Listed" means approved by UL.  The Personnel
Protection System for a "cord and plug connected EV" is a standard 120 VAC
Ground Fault Circuit Interrupter (GFCI).  GFCI "must trip" at 5 mA or less,
and often trip at 3-4 mA.  At this current level, they tend to false trip -
a 200 foot extension cord itself can have this much leakage.  The 5 mA
current level is based on one half of the AC "let go" threshold for a small
adult or child.  It's a good standard for toasters in the kitchen or hair
dryers in the bathroom.

If the tool, appliance or EV is well grounded, the probability of dangerous
leakage current traveling into a person is low.  A dry hand holding an
energized wire is 15,000 to 50,000 ohms.  A wet hand is 3,000 to 6,000 ohms.
A foot immersed in water is 100 to 300 ohms.  A good ground circuit should
be no more than 1-2 ohms.  Nearly all leakage current will travel back
through the ground circuit, if it's intact. 

Per NEC 305-6 (a) and (b) the requirement for a GFCI is waived at
construction sites if an "assured equipment ground" is in place.  This
exemption is allowed because of the false trip problem.  OEM  EV ran into
false trip problems at the 5 mA level, even with transformer isolated
switchmode chargers and AGM batteries.  Its due to inherent leakage at 312
to 324 volt levels and 5-10 kW charging rates.  So a new UL Personnel
Protection System was developed and listed; a 20 mA GFCI level plus a
"continuously verified ground".  This is what's incorporated into the EV
conductive charging stations such as the Avcon.  The 20 mA level is above
the "let go" threshold, but only about 1/3 of the heart fibrillation level.

Here's my recommendation;
*	The EV chassis must always be grounded during charging.  Very
important.  No two wire extension cords please.
*	If the charger is non-isolated, all AC and DC conductors and
terminals above 50 Volts must be insulated or covered during charging.
*	If the charger is transformer isolated, all AC conductors and
terminals above 50 Volts must be insulated or covered during charging.  It's
a good idea to also insulate or cover the DC conductors and terminals to
avoid shorts.
*	For extra safety, a GFCI should be used at the wall outlet (best
choice), in the cord or in the charger.  This covers the unlikely case of
someone reaching in and getting past the insulation.  Such as a child with a
green stick or piece of wire.  Or what if you leave the energized extension
cord on the driveway all day while you're gone?  Also, the NEC calls for a
GFCI.  What would your insurance company say if your EV wasn't meeting code
and there was an incident?
*	If the GFCI false trips it may be time to thoroughly clean the
batteries, adjust the charging algorithm (reduce excess gassing), insulate
more of the battery terminals, improve the wiring, install a power contactor
in the negative lead to the controller or step up to an isolated charger.
If all these measures have been taken and the GFCI still false trips, it may
be time to move up to an Avcon type system.

Happy and safe motoring, Art Mander, Denver