Re: [PSES] safety 60950 and surge suppression circuits

[Richard Nute]

Hi Joe: On 5/31/2013 8:16 PM, Joe Randolph wrote: Hi Rich: Thanks for responding to my request for an explanation of the logic behind allowing SPDs across isolation barriers. Overall, the principles you outline seem reasonable if the equipment has a reliable earth connection.  I'm not yet convinced that these principles adequately address equipment where the SPD is not connected to a reliable earth.  I will try to illustrate with a simple example.  While my example will be based on equipment that has no connection to protective earth, I should note that I also have concerns about equipment that uses what I call an "unreliable earth," which is an earth connection obtained solely through the ground pin on a Type A plug.  However, to keep things simple, I will not address that case here. Since I work mostly with telecom equipment that has to comply with clause 6 of 60950-1, I will focus on how clauses 6.1.2 and 6.2 address the placement of an SPD across a required isolation barrier.  A typical example might be a fax machine that uses a class 2 power supply with no connection to protective earth.  This fax machine connects to a phone line and also connects to a computer via a USB port. Clause 6.1.2 requires 1500 VRMS isolation between the phone line and the USB port.  However, this 1500 VRMS barrier is allowed to be bridged by a 400 volt SPD.  So, in normal use, the effective isolation is 400 volts.  If the SPD fails short, the isolation is zero.  Since the equipment has no connection to earth, protective earth has no role in the operation of the SPD. Clause 6.2 requires a 1000 VRMS barrier between the phone line and accessible parts, and also between the phone line and the USB port.  However, these two barriers are allowed to be bridged by an SPD of any voltage whatsoever.  For purposes of discussion, let's assume the designer chose to use a 200 volt SPD.  So, in normal use, the effective isolation would be 200 volts.  If the SPD fails short, the isolation is zero.  Since the equipment has no connection to earth, protective earth has no role in the operation of the SPD. Okay.  If the USB port is connected to a grounded PC (for example), then the SPD is between the phone line and electrical earth (regardless whether the earth is reliable). If the USB is connected to a Class II (double-insulated PC), then the SPD is connected between the phone line and... an open earth connection.  In the event of a common-mode transient over-voltage on the phone line, then no current can pass through the SPD.  (Of course, there is some very small current due to the stray system capacitance to earth through the mains transformer.)    DISCUSSION My principal question is why a safety standard would go to the trouble of calling out an isolation barrier of 1000 or 1500 VRMS, and then immediately state that it is okay to bridge this isolation barrier with an SPD.  The rationale is that the SPD is expected to fail open.  In this event, the isolation barrier must withstand the transient voltage. SPDs are considered unreliable.  They will fail.  They can fail as a short-circuit, or as an open- circuit, or any value of resistance between the two extremes. In normal use, the effective isolation barrier is the breakdown threshold of the SPD.  So what is the point of specifying an isolation barrier and then allowing it to be defeated in normal use?  If the isolation requirement is trying to address a perceived safety hazard, why doesn't that hazard exist in normal use (with the SPD installed)? The hazard that is mitigated by the isolation barrier is that of a fault in the equipment across the isolation barrier to the telephone line.  Down the telephone line, an unsuspecting telephone serviceman is working on the line expecting only normal telephone voltages.  This isolation must be retained even in the event of a lightning strike on the telephone line that otherwise could damage the isolation barrier. So, we have three situations.  First, isolation between equipment circuits and telephone circuits to prevent injury to a telephone serviceman.  Second, preservation of that isolation in the event of a transient (lightning) voltage that could come into the equipment on the telephone line.  Third, in the event of an over-voltage on the telephone line, the SPD prevents circuit damage within the equipment (but the SPD is expected to fail open).  My theory is this:  At some point long ago, safety experts determined that bridging an isolation barrier with an SPD would be okay if the SPD was connected to a reliable earth.  Over time, this constraint (connecting the SPD to a reliable earth) got lost, and the SPD exemption found its way into requirements such as 6.1.2 and 6.2 that do not explicitly require any earth connection whatsoever.  So, even though the SPD is not connected to a reliable earth, it has somehow been allowed anyway.  I think this may be an oversight in the standard. It seems to me that the *only* technical justification for allowing an SPD to bridge an isolation barrier is if the SPD is connected to a reliable earth.  That explanation makes sense to me and seems defensible.  However, in the absence of this constraint , allowing an SPD to be connected across an isolation barrier does not seem to make any sense at all. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com Best regards, Rich - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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