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.
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. 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)?
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
Hi Joe:-
Sorry for the delay in my reply to your questions.
SPDs are used on mains circuits, both between the
poles and poles to earth. In this latter application,
the SPD is in parallel with basic insulation.
SPDs are also used on low-voltage external circuits
that are subject to transient over-voltages such as
antenna circuits and telephone circuits. The SPDs
are between the external circuit and earth, or
between the external circuit and the mains circuit.
In this latter case, for the purposes of a transient
over-voltage originating in the external circuit,
the mains circuit is a connection to earth.
Usually, within the equipment, these external
circuits are isolated from earth and from equipment
secondary circuits. Indeed, the standards require
such isolation (to protect personnel touching such
circuits downstream from the equipment).
But, because these circuits are low voltage, the
isolation system is not a safety isolation, i.e, is
not a basic insulation.
Nevertheless, because the external circuits are
subject to transient over-voltages, the isolation
system must withstand such over-voltages. Therefore,
the isolation system is subject to an electric
strength test. And, the isolation system can be
bridged by an SPD at the discretion of the designer.
The principle I described in my previous message on
this subject applies here. In the case of an open-
circuit failure of the SPD, the external circuit
isolation system must withstand the expected
transient over-voltages. Hence, a separate voltage-
withstand test of the isolation system without the
SPD.
While the equipment is not required to have reliable
earth, many of the requirements are excluded if the
equipment does have reliable earth.
The standard does not address the short-circuit or
low-resistance failure of an SPD that is connected
between the external circuit and the mains.
However, external circuits are normally isolated
from earth, so no transient current can flow in the
reverse direction, i.e., from mains to the external
circuit.
I suppose the SPD can bridge the isolation, i.e.,
from external circuit to secondary circuit. If
the secondary circuit is connected to earth, then
the SPD simply conducts to earth. If the secondary
circuit is not earthed, then the SPD conducts
through the primary-secondary capacitance to the
mains.
Further questions or objections?
Best regards,
Rich
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