551-20

[r chenevert]

I am a retired electrical contractor, living in Canada,and do not have a US code book.

Have an Airstream travel trailer (aluminum body) that I'm restoring. I'm starting to rewire the entire thing. It has 110 volt wiring and a converter to 12 volts for some lights, water pump etc. I have been told through the Airstream forums to NOT bond the chassis to ac ground. Also told TO bond the converter to chassis. This doesn't seem quite right to me. If a hot wire touched onto the chassis, without the chassis being bonded to ac ground, wouldn't that liven the chassis providing a serious shock hazard? The people at Airstream forums think I'm not very knowledgable electrical wise for asking such a question.

Someone at Airstream forums suggested reading chapter 551-20 IF I was a real electrician.

I don't have a code book, can someone there explain 551-20 to me?

Thanks, Retired Ron

 

[WorkSafe]

551.20

(C) Bonding Voltage Converter Enclosures The non–current-carrying metal enclosure
of the voltage converter shall be bonded to the frame of the vehicle with a minimum 8
AWG copper conductor. The voltage converter shall be provided with a separate chassis bonding conductor that shall not be used as a current-carrying conductor.
The intent of 551.20(C) is to reduce the possibility of damage to the power supply cord by large dc fault currents that may find their way back to the vehicle frame or battery through the ac grounding conductor of the converter. Metal enclosures of listed converters are provided with an external pressure terminal connector for this purpose.

 

[nakulak]

go here:

http://www.nfpa.org/aboutthecodes/list_of_codes_and_standards.asp?cookie%5Ftest=1

and you can register and read any nfpa codebook including the NEC.

 

[r chenevert]

Safety 551-20

Safety 551-20

Thanks Mark and Nakulak.

One more question. The enclosure of the converter gets bonded to the chassis with minimum #8. The converter gets bonded with a separate conductor. The converter gets grounded through the three wire cord back to the panel in the trailer which is grounded through the large cord from the trailer to the plug in the campground or wherever. Why not bond the trailer chassis directly? The experts at airstream forums say DO NOT bond the trailer panel to the chassis. I don't understand.

Retired Ron

 

[r chenevert]

551-20

Thanks to Mark and Nakulak.

One more question. Why not bond the chassis directly to the trailer panel? It gets bonded indirectly anyway doesn't it? Airstreams forums are adamant...do NOT bond the chassis to the panel,

Thanks.

Retired Ron

 

[r chenevert]

Thanks to Nakulak and Mark.

I tried this several times but am not too computer literate and don't think I got through.

One more question.

I have the enclosure bonded, have the converter bonded. have the converter grounded through the power cable to the trailer panel. This effectively grounds the chassis but if the converter becomes unplugged the connection is lost.

How does one ground (bond) the chassis?

Retired Ron

 

[LarryFine]

Ron, welcome to the forum!

I have been told through the Airstream forums to NOT bond the chassis to ac ground. Also told TO bond the converter to chassis. This doesn't seem quite right to me. If a hot wire touched onto the chassis, without the chassis being bonded to ac ground, wouldn't that liven the chassis providing a serious shock hazard?

The trailer should be wired ass if it was a sub-panel; the neutral-to-EGC bond is done at the source, and not within the trailer or its power cord. Mobile homes are wired the same way.

The chassis is connected to the EGC, but never to the neutral, just as with premises wiring. (Think 4-wire major appliance circuits, not 3-wire.) The DC wiring is as usual for vehicles.