It helps to think about the differences and reasons for an IT system. A normal power distribution system is earthed at its point of origination (e.g transformer or generator) to limit the voltage difference between earth and points in the system. For example you may have a single phase 120/240v with an earthed center tap, or maybe a 230/400V Y system with a earthed neutral, fed from a several KV primary system. One reason for the earthing is to prevent primary to secondary breakdowns from allowing the primary voltage to appear (for long) on the secondary distribution side where nothing is designed for such high voltages. Having the secondary distribution side earthed has lots of other benefits but also means that if you have an earth fault, you are likely to have a shutdown. That's where IP comes in. Since the system essentially floats, a single fault does not shut down the circuit. You can allow such faults and schedule repairs as desired. Other protection schemes are generally provided which guard against high voltage primary faults and alarms notifying you of single earth faults. Since earth faults might occur and be sustained for some time at any point in the system, you might have voltages above earth anywhere in the system that are as high as the phase to phase voltage of the distribution system. Therefore in an example European IT system, where you might expect a receptacle to have one pole (neutral) at zero and another at 230 volts above earth, you may find either pole at up to 400 volts above earth, even though the poles are still only 230 volts apart and the product is operating normally. That has different implications for the design of insulation systems between mains and earth. For an IT rated product, the mains insulation should generally be designed for the phase to phase voltage of the power system rather than the phase to neutral voltage, whether a Class 1 ( earthed) or Class II (double insulated) system.
Typical 120/240V TN system ——X X x————————————————————+ Phase @ 120 V to earth X x product X x @120V X x———+————————————————+ Neutral @ 0 V to earth X x +——earth X x X x————————————————————+ Earth fault trips overcurrent ——X and disconnects system Typical 120/240V IT system ——X X x————————————————————+ Phase @ 120 V to earth X x product X x @120V X x———+————————————————+ Neutral @ 0 V to earth X x Z Impedance to earth @ 0 V X x +——earth X x X x————————————————— Phase @ 120 V to earth ——X Typical 120/240V IT system with fault ——X X x————————————————————+ Phase @ 240 V to earth X x product X x @120V X x———+————————————————+ Neutral @ 120 V to earth X x Z Impedance to earth @ 120V X x +——earth and in alarm state X x X x——————————Earth fault Phase @ 0 V to earth ——X Bob Johnson ITE Safety <http://www.itesafety.com> Carpentier Kristiaan wrote: Hi Group, Does it make sense to test a Class II wall plug PSU for IT power systems as there is no PE pin on such a PSU? Some CB reports mention that testing for IT power systems is done and some don't. Best regards, Kris Carpentier Regulatory & Approvals - 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 emc-p...@ieee.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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This e-mail has been scanned by MCI Managed Email Content Service, using Skeptic(tm) technology powered by MessageLabs. For more information on MCI's Managed Email Content Service, visit http://www.mci.com. ______________________________________________________________________ - ---------------------------------------------------------------- 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 emc-p...@ieee.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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
