Hi Bob: Thanks for your thoughtful response. You said: The working voltage increase would tend to approximately double creepage distances, but again these are mains to earth distances, not mains to mains. This can impact designs though.
Creepage distance requirements are based on long- term deterioration of the surface. The time to repair an earthed IT system is quite short compared to the time for long-term creepage distance deterioration. So, despite the time to repair the IT fault, I feel the fault-condition time would be insignificant to the deterioration of the normal-condition IT creepage distance. For this reason, I argue that we would NOT need to increase the creepage distance for an IT system. Then, we can say that the electric strength, clearance, and creepage distance requirements for an IT system can be the same as for a TN system. Further discussion? Best regards, Richard Nute Product Safety Consultant San Diego From: Robert Johnson [mailto:john...@itesafety.com] Sent: Monday, October 08, 2007 10:48 PM To: ri...@ieee.org Cc: emc-p...@ieee.org Subject: Re: Class II PSU for IT power systems Rich, My view is a product for an IT power system should have its mains insulation selected based on the phase to phase voltage of the supply. While most designs are based on normal, not fault conditions for the product, the IT power system is faced with fault conditions caused by any or all products plugged into the power system. It is faced with enduring these conditions until repairs are made, a timetable which may be weeks or longer depending on the attentiveness of the maintenance staff. For neighborhood rather than in-house systems, maintenance may not be up to the owner. An IT power system fault is not necessarily an unusual condition. Note the increase of 1.7x in working voltage applies only to the mains to earth insulation, not across mains ( phase to neutral). This increase would have no effect on transient voltage ratings (and therefore clearances) in IEC 60950 since in Table 2J the 150 volt row includes phase voltages up to 240 volts and the 300 volt row includes phase voltages up to 480 volts. That is, the transient voltages expected do not depend on whether you are looking at single or three phase products connected to that power system. The working voltage increase would tend to approximately double creepage distances, but again these are mains to earth distances, not mains to mains. This can impact designs though. However, standards have not specifically addressed design requirements for products intended for IT power systems, and some countries like Norway have expressed no need for increased spacings in products for their IT systems. >From a safety standpoint I find mains to earth insulation breakdown rare. It is usually a total bypass such as cut insulation, direct shorts, etc. rather than spacing violations or dielectric failures which cause such faults. I would not expect to see a difference in product failures based on their use in IT power systems. You mentioned open neutral conditions. This is likely to be a much more significant risk of hazardous failure. While many products will work or safely fail at undervoltage conditions, few have been designed for 1.5x or more overvoltage operation. Bob Johnson ITE Safety <http://www.itesafety.com> rn...@san.rr.com wrote: Hi Bob: Very nice discussion of the salient features of the IT power distribution system. Somewhat related to the original question, should the product mains insulation requirements be based on normal operation of the IT power distribution system or based on the worst-case fault of the IT power distribution system? As a general rule, basic insulation requirements are based on normal operating conditions, not on single-fault conditions. For example, we do not require the product to include safeguards against open-neutral conditions on a three-phase system. Except for creepage distances, the product basic insulation is a function of the mains transient overvoltage. In an IT power distribution system, I don't believe that earthing of any pole changes the magnitude of any transient voltage. However, I have not studied this point. If the transient voltages do not change magnitude, then the product basic insulations, in the event of a fault, are not subjected to any higher transient overvoltage than under normal conditions. If this is the case, then the normal-condition basic insulation is functional as a safeguard even under single-fault conditions of the IT power distribution system. Even if I am wrong about this, the topic I want to discuss is whether basic insulation requirements should be based on fault-condition voltages in an IT power distribution system. As a general rule, basic insulation is expected to fail, and either supplementary insulation or earthing provides the safeguard. By definition, supplementary insulation and earthing provide a safeguard function in the event of failure of basic insulation. Discussion? Regards, Richard Nute Product Safety Consultant San Diego From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Robert Johnson Sent: Saturday, October 06, 2007 2:27 PM To: owner-emc-p...@listserv.ieee.org Cc: emc-p...@ieee.org Subject: Re: Class II PSU for IT power systems 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 <http://www.itesafety.com> Safety ______________________________________________________________________ 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
