Many years ago I helped CSA write TIL I-35 which covers automotive inverters. We did the drawings that John Woodgate suggests, and we analyzed shock hazard potential under normal conditions and with a single fault in the inverter or in the load appliance. We included Class 1 and Class 2 products in our deliberations. As it is nearly universal, we assumed that the car's battery had one side grounded to the car chassis. The result was that there are several ways to make such inverters safe.
The easiest is to ground the inverter chassis and the inverter output receptacle ground slot back to the DC negative, creating an equipotential system including the car chassis, the inverter chassis, and the chassis of all Class 1 load appliances. If you do that, whether or not the inverter provides isolation becomes moot. The insulation/isolation approach was also allowed, whereby the inverter would be tested to see how much leakage current could flow from the AC output line or neutral back to DC + or -, and if the answer was less than a shock hazardous value, then the inverter isolation was deemed to be adequate protection against shock hazard due to ground faults in the load appliance. The isolation in the inverter transformer and between the inverters AC output circuits and the inverter chassis could be Basic because faulting that insulation would only pose a hazard if another simultaneous fault was present and we weren't going for double-fault safety. In this approach, the provision of an output ground slot, and what it's connected to, become moot. Another option allowed was to add ground fault protection to the AC output in which case isolation and grounding become moot. The worst thing you can do is to provide no isolation or inadequate isolation in the inverter and provide a ground slot on the output receptacle that connects to nothing. On small inverters, the provision of an external ground stud on the chassis is useless because nobody will ever connect it to anything. Ever. On larger inverters meant for RV's (caravans), such a stud is required by UL458. There are many low cost automotive inverters on the market that do not adequately address these considerations. Jim Eichner, P.Eng. Compliance Engineering Manager Xantrex Technology Inc. phone: (604) 422-2546 mobile: (604) 418-8472 e-mail: [email protected] web: www.xantrex.com Confidentiality Notice: This email message, including any attachments, is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies of the original message. From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Wednesday, March 18, 2009 11:27 AM To: 'Haynes, Tim (SELEX GALILEO, UK)' Cc: [email protected] Subject: RE: Intersting question!! > What is the isolation between the DC and AC circuits? No safety isolation is necessary between the automobile DC and the inverter output AC. Indeed, for most inverters, safety isolation would make the inverter much larger and more expensive. Inverters generate plus and minus high voltage steps with respect to the DC. The steps approximate the energy-time content of a sinusoidal waveform. Such circuits cannot have one AC pole connected to the DC common. The AC output socket should be an earthing type. The earthing terminal of the socket should be connected to the common pole of the car's DC. This creates the equipotential environment with respect to the metal of the car. Best regards, Rich - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
