Rick, Unless you meet NEC 250.21, you are stuck with grounding one of the supply conductors, if your transformer is part of your building wiring installation, and not evaluated as part of the equipment in accordance with UL 60950-1. Read NEC 250.21 to see if you fall into one of the categories of "alternating-current systems of 50 Volts to 1000 Volts not required to be grounded", subitem (3) concerning separately-derived systems supplied by transformers that have primary voltage rating less than 1000 volts. To comply with this section, all of the following conditions must be met: a. The system is used exclusively for control circuits <<this seems to be your case>> b. The conditions of maintenence and supervision ensure that only qualified persons service the installation <<this must be the case>> c. Continuity of power control is required <<??>> d. Ground detectors are installed on the control system<<this must be the case>> By doing this, you effectively create an impedance grounding system (see NEC 250.20(E)), or what the IEC calls an "IT" power distribution system. In the US, this is only permitted when qualified supervision is assured. The exception to grounding the supply conductor is granted for systems where a ground fault removing power would be catastrophic, such as with an industrial crane, or as you noted, life-support medical devices, or industrial furnaces, which would not work, I assume, if grounded. In your case, you would have the extra burden of ground detectors and supervision, which may be more than you wish to undertake. If your transformer circuitry is such that it can be evaluated as part of your Listed ITE, you would have to have a system of reinforce insulation throughout the secondary in accordance with UL 60950-1, and all else that goes with it. Regards,
Don Gies, N.C.E Senior Product Compliance Engineer Lucent Technologies - Bell Labs Holmdel, NJ 07733 USA From: [email protected] [mailto:[email protected]]On Behalf Of Rick Busche Sent: Friday, December 17, 2004 9:04 AM To: [email protected] Subject: Power Transformer - Grounded Secondary An associate of mine is working on simulation training system that includes a motion platform and a mainframe computer graphics system. In order to meet conducted noise from the actuators in the motion platform, he has chosen to install isolation transformers on the single phase 120V line. The problem is that according to publications FIPS-94 and NEC article 250-30, (was article 250-26 in the 1996 code) a leg of the secondary must be grounded for safety. This is apparently required for all separately derived power systems. Now I understand that a PDU or PDS (transformer) as a separately derived power source is required to be grounded, but are we mixing apples and oranges here. In the instance of the simulator, this transformer does not provide power to anything but the simulator motion actuators themselves. 1. Would it not be reasonable for an ITE application such as this to maintain an isolated output for purposes of both noise and safety? I frequently find medical equipment with a requirement for isolated ac power. 2. If this is required is applicable is there a work-around on this? Thanks in advance… Rick Busche - Product Safety Engineer ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/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] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/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] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
