How to determine the ratings of components in primary circuit

Roland chung

Trusted Information Resource
#1
Hello forum,

The standard just says the components including wiring shall be used in accordance with their specified ratings, but it is unclear how to determine the ratings.

For the current rating of components in primary circuit, three options swim into my mind:

1) not less than the current rating of primary protective devices, such as mains fuses, circuit breaker

2) not less than the rated current of equipment

3) not less than the current that components are intended to carry when the equipment is operating under normal load

Which one mentioned above do you think is correct? Can I have your opinions?

Many thanks,
Roland
 
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Peter Selvey

Staff member
Super Moderator
#3
In general it's the normal condition of use, unless the part is required to keep certain characteristics also under abnormal/fault conditions.

For example, equipment rated 2.5A, actual input is 1.7A, with a fuse of 3A, has an EMC inductor rated for 2A. It's generally OK because 2A > 1.7A, and because under abnormal condition there is usually no safety issue with an inductor overheating.

However, if the inductor happened to be near double insulation, then rating might need to be checked also in abnormal condition.
 

Roland chung

Trusted Information Resource
#4
That means component which is safety relevant should be considered under normal conditions and fault conditions. Right?

Is it ok for high capacity equipment rated 100A, actual input is 30A, with a circuit breaker of 125A, has a terminal block rated for 50A?
 

Peter Selvey

Staff member
Super Moderator
#5
For the example shown, clause 4.8 of IEC 60601-1:2005 should be Pass. If it were not, we would be in a situation where all components after over-current protection (fuse or circuit breaker) would have to be rated the same or higher than the over-current protection, which is not practical.

However, if there is a large difference between the rating of a component that carries fault current and the rating of the over-current protection, there may be doubt about suitability in fault condition. It's not a non-compliance, but tests may be necessary under 8.11.6 b).

For example, a 100A circuit breaker protecting a 2A terminal block would clearly require a test, in the test we could expect the terminal block to burn out or even explode before the breaker trips, potentially damaging insulation, starting a fire or scaring the pants off the test engineer.

A 100A breaker and 30A terminal block is probably OK. To keep the temperature rise down, a 30A terminal would need to have less than 0.001ohms contact resistance, which is negligible in the overall circuit including under fault conditions (in contrast, a 2A terminal could have 0.1ohms contact resistance, which is relatively large).
 

Roland chung

Trusted Information Resource
#6
For the example shown, clause 4.8 of IEC 60601-1:2005 should be Pass. If it were not, we would be in a situation where all components after over-current protection (fuse or circuit breaker) would have to be rated the same or higher than the over-current protection, which is not practical.
But the point in 60601-1:1988 is that the current rating of over-current releases shall not be greater than the current rating of any component in the mains circuit carrying the mains supply current. We can find the statement in clause 57.6
 

Peter Selvey

Staff member
Super Moderator
#7
It seems this requirement was removed in the 3rd edition. In the 2nd edition the text is any part "carrying the mains supply current" but that is undefined, imprecise and doesn't make sense for a number of reasons.

To comply with the second edition, you could fall back on Clause 4.1, which states that requirements are only applied if there is some safety relevance.

But of course it could be difficult to convince a test lab to agree :D
 

Roland chung

Trusted Information Resource
#8
Although there is no a definition for "carrying the mains supply current", it seems obvious. I would think it is not a valid arguing point.

Regarding the test to verify the suitability of component, the test lab said type test is always not sufficient when it comes to the component.
 

Peter Selvey

Staff member
Super Moderator
#9
It's obvious in some cases, like a single circuit that has only supply leads, terminal block, switch and a heater.

But what if there are branch circuits? Do you apply to each branch? If yes, each branch will then require a separate fuse according to the parts in that circuit. If no the requirement makes no sense (why apply just to the "main" circuit? It's inconsistent).

But if you apply the "fuse each branch" rule, then any parallel component (capacitor, diode bridge, bleed resistor etc) becomes it's own mini branch circuit, which would require a fuse ... it would be endless and impractical.

The real situation is that that most small parts don't rely on the fuse to protect against fire. Rather, the small size of the part limits the fault energy, they tend to burn out on it's own, and any fire will be limited through the use of non-flammable materials (PCB, wiring, enclosure etc). I once tried to start a fire using tissue paper and an overloaded 0.25W resistor, it was impossible.

For high current parts (10A or more), I-squared-R power loss means the resistance has to be very low just to be functional. It's not really an issue for fire caused by overcurrent, unless the fuse size is >> rating (e.g. 10 times higher).

But there is a sweet spot in between where neither of these apply. A large bulky part rated for 2A protected by a 10A fuse could be trouble in fault condition. This happens some times with high power resistors (intended for inrush or dumping capacitors), exceeding 400C in SFC for extended periods.

If in doubt, do the test.
 
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