Failure of X-Capacitors complying with IEC 60384-14

Roland chung

Trusted Information Resource
#1
Dear all,

In IEC 60601-1:1988/A1, capacitors (X1 and X2) complying with IEC 60384-14, which are connected between Line and Neutral, are exempted from the fault tests (e.g. short circuit).

However, this exemption does not exist in IEC 60601-1:2005 any more. Does this mean that failure of such capacitors can be simulated?

The question proposed since there is a switching power supply that X1 capacitor is before the mains fuse. If short circuit of X1 capacitor is simulated, this design will cause the branch breaker to operate and therefore fail the standard.

Regards,
Roland
 
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Peter Selvey

Staff member
Super Moderator
#3
It is desirable to avoid causing the branch fuse to open, but I don't think it's a requirement (correct me if I'm wrong). If you assume that a SFC cannot result in the branch fuse opening, it implies that double/reinforced insulation is required L-N before the fuse.

The L-N creepage distance before the fuse, for example, is only 3mm, which is not double insulation. I think the intention is to reduce the probability of faults before the fuse, but not to the extent of double insulation. So an X-capacitor is not double insulation, but it still has a pretty low probability of faults.
 

Roland chung

Trusted Information Resource
#4
It is desirable to avoid causing the branch fuse to open, but I don't think it's a requirement (correct me if I'm wrong).
The requirement is 8.9.2 a). The rationale clearly indicates that opening of a branch circuit breaker is not acceptable.

Yes, only 1 MOOP is required L-N before the mains fuse. I agree that 60384 approved X-capacitors provide 1 MOOP. But if there is a bleeder resistor in parallel with the X-capacitor, would this design be acceptable?

Roland
 

Peter Selvey

Staff member
Super Moderator
#5
8.9.2 a) - A quick glace at the clause and rationale implies that opening the supply protection in SFC is bad. But on careful reading, it simply says when to apply Table 11 (just cr/cl), nothing to do with components or SFC. And the values in Table 11 are less than 1 MOP.

Better to look at 8.8.1, which says that only 1 MOP is required before the fuse. And it says that components meeting relevant IEC standards (4.8) don't need to be tested. So, clearly the X-type cap is OK. If the bleed resistor is part of an approved component (e.g. EMC filter), it can also be treated as OK, but if the bleed resistor is part of end product, it is reasonable to check. For example, a single SMT1608 resistor is clearly no good, because a string of 3 or 4 SMT resistors should be fine. Two 0.25W metal film resistors should also be OK. Because the resistor itself is likely overheat with 1500V applied, for an actual test might need to replace the resistor with the same type but much larger resistance (e.g. 10Meg) or just apply some common sense. Of course, if it is in the end product, it's just as easy to put the bleed resistor after the fuse, to make it simple.
 

Roland chung

Trusted Information Resource
#6
One more question about the X-capacitors.

Can X-capacitors be used between Line and Earth? The equipment is permanently installed, so the leakage current is not a concern.
 
#7
For line to earth you need Y class caps.

Where this is across a Means of Patient Protection (MOPP) it needs to be Y1.

For MOOP it can be Y2.

Note that 1 capacitor is considered to provide 1 MOP; to provide 2 MOPP (or MOOP) needs 2 in series each rated for the full voltage.

See clause 8.5.1.2 and 8.5.1.3
 

Roland chung

Trusted Information Resource
#8
The standard just says a Y capacitor (Y1 or Y2) complying with IEC 60384-14 is considered equivalent to one MOP. But there is no statement such that capacitor used between line and earth MUST be Y type.
 
#9
The requirement for Y caps for L-E connection comes from IEC 60384-14; the standard for X and Y capacitors.

Quite often a single capacitor can be rated both as an X and a Y, but at a lower voltage rating for the more stringent Y class.
 
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