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Posted By Topic: 10mA RCD with 40ms maximum trip time.

Sep 14 2017 00:23

Can anyone explain why an RCD with a 10mA sensitivity must trip at a maximum time of 40ms, while an RCD with a 30mA sensitivity has a maximum trip time of 300ms?

According to the chart found in IEC 60479: Effects of current on human beings and livestock, the greater the amount of current passing through the body the shorter the duration before serious harm can occur.

For example:
1) 230mA of current passing though the body becomes seriously harmful at 40ms duration.
2) 30mA of current passing through a body becomes seriously harmful at (approx.) 400ms duration.
3) 10mA of current, according to the graph, does not even enter zone 3 (zone which signifies serious harm can result)

So the curve shows that with greater current (obviously) less time is afforded before serious harm is done.

So why limit a 10mA RCD to a much quicker trip time? Shouldn't the higher sensitivity mean that it is safe to allow it to trip at or greater than an RCD with 30mA sensitivity?

Sep 14 2017 04:25

Because the 10mA units are used typically in patient areas and areas where there are young children both of which are more likely to suffer more harm in the event of an electric shock

Sep 14 2017 06:24

Yes, agreed, the use of 10mA in areas such as this gives greater sensitivity.

But what I am getting at is if 10mA is essentially not harmful, is the important factor duration?

And if so, why not simply state that in medical areas the duration limit is, say, 40ms? (With no reference to mA)

Sep 14 2017 10:21

Both the level and the duration of shock current are relevant. The curves on the graphs in "60479" show the relationship between current level and duration.

Broadly speaking,30 mA is related to / based on onset of ventricular fibrilation.
10 mA is related to / based on threshold of let-go - which is why RCDs for schools must be 10 mA not 30 mA.

Not all 10 mA RCDs have to trip within 40 ms @ rated residual current - Type I RCDs for medical do, but the Type A ones we use for schools have the same 300 mS trip time as 30 mA. When you get up to current level 5 x rating, both Types must trip within 40 ms.

Most of the resistance of a body is in the skin - we're basically a big bag of highly conductive salt water. For general purpose RCds, we're using them as an extra defence in situations where contact with the live stuff is already unlikely. For medical, we're dealing with situations where contact is often part of a medical process; we're likely to be dealing with people who have less-than-normal body resistance 9contact inside the skin; and who have less-than-average ability to withstand shock (not healthy adults). We want to be as sure as we can be that if anything goes wrong, we stay out of the "serious harm" zone of those graphs.

Also relevant is the waveform of the fault current, which is why NZ requires RCDs capable of tripping on pulsating d.c. (Oz doesn't). However in some cases even these are not good enough; ie may well not trip under fault conditions because the fault current's waveform is beyond the sensing ability of the RCD. That's why we're going to have to use Type B for EV charging, as Type A may well not trip.


Sep 14 2017 13:34

Thanks AlecK I totally agree, although I wasn't aware of the increased limit (300ms) for type A 10mA RCDs.

Curious if you have any practical feedback on the arrangement used in schools.
Are 10mA RCDs installed in switchboards resulting in nuisance tripping? If so, is it preferred to install socket RCDs?

Sep 14 2017 15:01

Yes 10 mA RCDs have been known to trip when installed at switchboard; usually due to "standing leakage" on the subcircuit. Pretty much every circuit has some leakage, much of it due to capacitance. There;s also leakage from appliances, and reducing the number of sockets per RCD can avoid tripping from this cause - which is the reason for the IT Exception.

So yes, preferable to have the 10 mA RCds close to the sockets they are supposed to protect. either SRCDs or separate RCD adjacent.
Note that the wording of requires protection of "socket-outlets"; whereas in it's protection of "final subcircuits". The clauses are worded differently for exactly this reason.