DC-GFP/2 protection [RE-wrenches]

[Rob Wills]

Two comments on the DG-GFP protection issue:

1/ Grounding the negative in PV systems, or at least bonding it to the case
of the electronics is essential.  I did the East Coast service for Heart
Interface in the early 80's.  Every August 5-10 inverters came in with
lightning damage.  When asked "is the dc grounded" the customer invariably
said "no".  And it's not the grounding that saves the inverter - it's
bonding inverter case to DC negative. (connecting case to ground serves
another purpose - making sure that the customer does not get fried..)

Inside the inverter, all electronics is referenced to batt/pv negative.  A
lightning induced voltage on unbonded PV wiring can result in a huge voltage
between circuit boards and case - and will cause inverters to fail.

Something else to understand is that it does not take a direct strike to
take out an inverter.  The PV system is sandwiched between the plates of a
large capacitor (ground and the lightning bolt) and/or can pick up current
by magnetic induction in PV source circuits.  In large systems, always
minimize your source circuit loop area by panel layout and crossing over the
string wiring between runs of modules.

2/ The requirement for separate disconnect and GFP trip breakers is an old
issue for us.
Years ago, we introduced the GC-1000 inverter that uses the same breaker(s)
for disconnect and GF trip.  UL accepted it, but John Wiles questioned it a
few times.  We referred John to NEC Section 250-4(A), which is referenced in
690-41 (the section that specifies the grounding requirements).  The MM-5000
inverter used the same scheme and is also approved by UL.

A key phrase in 250-4A is (A)1: Electrical systems that are grounded shall
be connected to earth in a manner that will limit the voltage posed by
lightning, line surges, or intentional contact with higher-voltage lines,
and that will stabilize the voltage to earth during normal operation.

So the requirements are here, as is the phrase "during normal operation".
We maintain that the only time that the disconnect/GFP will be opened is for
service, and that this is not "normal operation".

There's more - 1/ if you are doing service and hunting for a ground fault,
you need to unground the array.
2/ Any working on an array is likely to turn of ALL breakers, because it
looks safer to have all those switches down - the additional breaker will
not be used for the purpose that it was put there anyway
3/ if an installer wants to disconnect a positive only, when chasing a
fault, they can pull a string fuse.
4/ The extra breaker or switch adds unnecessary cost and another element of
unreliability.

This is a case where common sense went out the window and was replaced by a
rigid, but in my view, improper, interpretation of the code.

Regards

Rob Wills
VP Engineering
Advanced Energy

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com

==^================================================================
This email was sent to: michael.welch at homepower.com

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9
Or send an email to: RE-wrenches-unsubscribe at topica.com

T O P I C A -- Register now to manage your mail!
http://www.topica.com/partner/tag02/register
==^================================================================