Your explanation makes sense but it presumes the SPD shunts the current to ground.
It would appear to me that the SPD is allowed to simply bridge the isolation barrier. That is the aspect that seems contradictory to me.
In 60950-1 clauses 6.1.2 and 6.2 (the ones I work with most frequently) there is no explicit requirement that the SPD be connected to a reliable ground. Rather, it is simply allowed to bridge the isolation barrier. Based on quick read of clause 5.2.2, the situation would appear to be similar there as well.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
Throwing in my 2 cents:-
Hipot test values are based on expected transients. The concern with the transient overvoltages is that they could punch through insulation needed for safety and subsequently hazardous voltages can be allowed to reach areas where they should not. The function of an SPD is to limit the level of transient overvoltages that propagate through the equipment. If the SPD functions correctly, the insulation should not be stressed by the transients since the SPD shunts the surge current to ground. In other words, the function of the SPD makes it different from the other insulation barriers. While the other barriers must withstand the transient overvoltages, the SPD functions to limit them. Because of the difference in function, it is appropriate to test them differently, or to treat them differently during test.
As Rich stated, an SPD may not be reliable in its function to limit the transient overvoltages (for example SPDs are often themselves protected by fusing which then leaves the SPD function inoperable if the fuse opens), so it is appropriate to test the rest of the insulation as if the SPD were not present.
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From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Joe Randolph
Sent: Tuesday, May 21, 2013 3:41 PM
To: ri...@ieee.org
Cc: Bill Owsley; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: safety 60950 and surge suppression circuits
Hi Rich:
The apparent contradiction that I was trying to describe can be summarized as follows:
a) The safety insulation barrier must withstand a hipot test of, say, 1500 VRMS
b) It is permissible to bridge this safety insulation barrier with a SPD that breaks down at, say, 300 VRMS
In other words, in normal use, the safety insulation barrier can be bypassed by a SPD. For the seemingly artificial circumstances of the hipot test, the SPD can be removed in order to demonstrate compliance with the 1500 VRMS requirement. The 1500 VRMS safety isolation barrier would appear to be relatively useless if, in normal use, it is bridged by a 300 VRMS SPD.
In the above example, I avoided specific reference to particular clauses in 60950-1, because I wanted to illustrate the basic scenario that seems to recur in a variety of places, but with differing details. In 60950-1, this scenario seems to appear in clauses 5.2.2, 6.1.2, and 6.2. It also appears in TIA-968 for equipment connected to the telecom network.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
Hi Joe:
Very quickly...
SPDs are not considered reliable components or assemblies.
The safety standards anticipate a failure -- anywhere from
open-circuit to short-circuit.
In the event of an open-circuit, there is no indication of
such a failure. And, of course, all transients then pass
through the open SPD.
Consequently, the equipment safety insulations will be
called upon to withstand the expected transient overvoltages.
So, the standards require performing the voltage withstand
test without the SPD in place.
Best regards,
Rich
On 5/20/2013 1:40 PM, Joe Randolph wrote:
Hi Rich:
I'm hoping that you can provide one of your straightforward "Rich Nute Explanations" for the apparent contradiction behind the rationale that allows a surge protection component to be placed across a required safety isolation barrier, and then removed for the purpose of performing the hipot test.
I have been involved with safety compliance for over 30 years, and this concept is one that has never made complete sense to me. On one level, I can just bump along and limit my attention to what the safety standard actually says, but I would like to understand what the thinking is behind that.
This allowance (removing surge protection components for the hipot test) appears in a variety of standards and clauses within those standards, such as EN 60950-1, clauses 5.2.2, 6.1.2, and 6.2.2.
If you could help clarify the thinking behind this allowance, I would greatly appreciate it.
Thanks,
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
Hi Bill:
SPDs, regardless of configuration, are notorious for being
prone to failure, either short-circuit or open-circuit or
any value of resistance between those two extremes. (One
cannot predict the energy the SPD will be required to
dissipate.)
From a safety point of view, all such failures must be
accounted for such that the safety of the equipment is not
compromised by any failure of the SPD.
If the SPD should fail open-circuit, then expected
transients that are therefore not attenuated, must not
cause the insulation to fail. Hence, the insulation must
pass the hi-pot test without the SPD in place.
As for the requirement for the GDT to pass the hi-pot test...
??? I don't have any rationale for this.
Best regards,
Rich
On 5/10/2013 10:11 AM, Bill Owsley wrote:
I'm running into a dilemma. Not being a Safety Engineer myself, but rubbing elbows with them...
On a piece of ITE equipment, I need some surge suppression for worldwide markets with one annoying requirement for 4 kV, otherwise just 2 kV line to earth, and using either plugable cords or permanent connection, whichever is worse.
Now the Safety guys tell me that MOV's alone cannot bridge the insulation (Basic or Functional, I forget.) between primary and earth, when using one of power cable options mentioned above.
But a proper qualified (GDT) gas discharge tube can do the bridging. So we figured to use them in series.
On a quick and dirty bench test it works to 4 kV.
Then the Safety guys pull out the rest of the story and point out 5.2.2 which seems to indicate that the GDT is to meet the Hi-Pot test, 1500 vac.
Previously, section, 1.5.9.4 (?) indicates that the surge protection devices can be removed during the Hi-Pot test.
But now I have a Surge suppression circuit that has to withstand the same Hi-Pot as the rest of the board.
Question is how does a surge protection circuit protect the board when it has to meet the same Hi-Pot test?
In other words, when a surge comes along, which is going to break over first?
The surge protection or the board?
Is the purpose of surge suppression is to keep the clamped voltage below a problem level?
What am I missing in this?
Thanks...
- Bill
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Joe Randolph
Telecom Design Consultant
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781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
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