Thanks for insight on this. I think your remarks, copied below, explain the logic behind clause 6.1.2, but still leave questions about the thinking behind clause 6.2.
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).
CLAUSE 6.1.2
Clause 6.1.2 is the one that addresses the problem of hazards within the equipment getting onto the phone line and injuring a telephone service person who is working on the network. I think the origin of this requirement comes from the old UK standard BS 6301, and was based on the possibility that a mis-wired mains plug could result in the equipment ground wire being connected to a live mains wire (this fault mechanism is more common in the UK than in most other countries due to the way consumers deal with conflicting plug configurations). So, in this case, an equipment chassis that is supposed to be grounded becomes "hot."
Clause 6.1.2 requires that any SPD connected across the barrier have a breakdown threshold that would not turn on for normal mains voltages. So, for clause 6.1.2, I would summarize your explanation as follows:
1) The actual goal of the requirement is to always maintain a barrier that will not break down for normal mains voltages (about 400 volts peak for Europe) .
2) If there is no SPD across the barrier to protect the barrier from lightning damage (or if the SPD is present but has failed open), the barrier must withstand an expected worst-case lightning surge of about 2100 volts peak (verified with a 1500 VRMS hipot test).
The above rationale is interesting because it says that the purpose of the SPD is to protect the isolation barrier. This makes sense except that it ignores that possibility that the SPD could fail short. If all SPDs were gas tubes, ignoring the fail-short possibility might be a reasonable assumption, but with the solid state SPDs that are now in common use, a fail-short mechanism is actually more likely than a fail-open. In fact, many solid state SPDs are explicitly designed to fail short when overstressed. MOVs, which have been used for many years in various telecom protection circuits, also have a fail-short mechanism that is probably more common than fail-open.
CLAUSE 6.2
Clause 6.2 is concerned with protecting equipment users from hazardous voltages that may appear on the phone network (presumably lightning and power cross). In this clause, a 1500 VRMS barrier is called out for parts of the equipment that are hand-held. Significantly, SPDs placed across this barrier may not be removed during the test. This makes sense as long as the only possible failure mechanism of the SPD is fail-open.
For other parts of the equipment, a 1000 VRMS barrier is required, but this barrier is allowed to be bridged by an SPD of any voltage whatsoever. It is allowable to remove the SPD for the 1000 VRMS test. In this case, I do not see what value the 1000 VRMS barrier has if, during normal use, the barrier can be bridged by an SPD.
SUMMARY
Your explanation of the rationale used in the standard makes sense for clause 6.1.2 (protection of service personnel) as long as the only possible failure mechanism of the SPD is fail-open. However, I think the assumption that SPDs can only fail open is flawed.
For 6.2 (protection of equipment users), I do not see how the 1000 VRMS barrier provides any degree of safety protection if it can be bridged by an SPD in normal use. This still makes no sense to me.
Do you think it is possible that the theory I advanced in my last posting is at the root of this discrepancy? Namely, that the authors of the standard inadvertently assumed that all SPDs are connected to a reliable earth, which would make it okay to have them in place? The problem I see is that clause 6.2 does not require that the SPD be connected to a reliable earth. The SPD can simply bridge the required 1000 VRMS barrier, which effectively defeats the barrier during normal use.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
Hi Joe:-
On 5/31/2013 8:16 PM, Joe Randolph wrote:
Hi Rich:Okay. If the USB port is connected to a grounded PC (for example), then the SPD is between the phone
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.
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.)
The rationale is that the SPD is expected to fail open. In this event, the isolation barrier must
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.
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
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