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How to understand the IEC 60601 50 ms limit?

Roland chung

Trusted Information Resource
#1
Hello folks,

Either 2nd Ed or 3rd Ed, there is a statement saying that transient currents occurring during the first 50 ms following the short circuit shall be disregarded. This means, the currents shall not exceed the allowable value for SFC after 50 ms. It can also imply that the protection (e.g. fuse, regulating network) in place shall operate within 50 ms.

When we conducted a SFC (high voltage shorted to earthed enclosure plus PE interrupted because of the spacing is less than 1 MOP), the protection system took a little more time than 50 ms to operate and the touch current exceeded the limit before the operating. It seems to fail the standard clearly.

However, from the risk analysis, it is a very unlikely case (HV shorted to frame and PE interrupted à one touches the enclosure before the operation of protection system (millisecond field) à hurt or death). I would think the 50 ms limit can be skipped except for the application of heart.

What is your opinion? I would appreciate your thoughts.

Roland
 
Last edited:
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Peter Selvey

Staff member
Super Moderator
#2
The risk of electric shock, applied to the body externally, depends on the open circuit voltage, available current and time.

So to make a decision you would need to know all three factors.

I have an old Australian standard AS 3859 which has a diagrams which allow you to work out the probability of adverse effects based on these factors.

For example, if the open circuit voltage is high (>100V), the current is 50mA, it looks like about 300ms would be a "safe" limit for body current (not direct to the heart).

But if the current is 500mA, even 10ms would be dangerous.
 
#3
And IEC 61201, (Touch Voltage threshold values for protection against electric shock) has lots of data relating harms (fibrillation, muscular reaction, burns) to current path, time, current, AC/DC, skin condition (dry/wet) and many more.

It is a Technical Specification standard used to develop safety aspects of other standards.
 
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