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Posted By Jonf
While portable RCD`s only operate with class 1 equipment, is the only other protection to class 2 to PAT test? with visual (user) inspections.
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Posted By Ron Hunter
RCDs do operate with Class 2 appliances.
The function of the device is to detect an imbalance across live and neutral current flow.
RCDs are the ideal safety device in a domestic setting where class 2 lawnmowers, hedge trimmers etc are commonplace.
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Posted By Jonf
Many thanks Ron for your response, however, all your examples are class 1 (earth cable supplied). The RCD will work with the above; but class 2 has no earth cable (double insulated) - therefore "protection" from the RCD is nil.
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Posted By Jonf
Further - not domestic environment, construction!
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Posted By Tony Priest
Jonf
I think that you are getting confused with Residual Current Device and and Earth Leakage Circuit Breaker. A RCD will give protection when used on appliance without an earth circuit. As Ron has discribed RCD will detect an imbalance across live and neutral current flow.
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Posted By Ron Hunter
Jonf is also confused about the classification of everyday appliances.Doemstic lawnmowers, hedge trimmers etc. are NOT Class 1 appliances.
From a construction perspective, not uncommon to see RCDs "built in" to 110V CTE transformer sets.
Rest assured Jonf, these do work on construction sites for class 2 appliances.
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Posted By Alan Hoskins
Jonf,
The only situation where an RCD will not provide protection is if you bridge between line and neutral directly and there is no leakage to earth.
Any leakage to earth (above the RCD trip current) will operate the RCD with or without an earth conductor.
The reason for using these devices is to prevent dangerous currents from flowing through a person's body under fault conditions.
Alan
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Posted By mark gough2
Alan is correct an RCD will not operate on a direct short circuit. In practise a direct short will be detected by an overcurrent device and the circuit will trip.
Say the cable is severed leaving two exposed conductors this may not be detected by any device. If someone then touched the exposed conductors they would become the earth conductor and receive an electrical shock. The level of earth fault current flowing through the person should be sufficient to to operate an RCD.
In practise when a cable is severed it is often in contact with grass/ damp ground etc this would provide a sufficient earth path to trip an RCD.
In practise for the majority of faults it is irrelevant if equipment is class 1 or 2
The earth on class 1 should protect against the case remaining live if a live conductor touches the case and is an important safety feature.
It must be remembered an RCD only offers supplementary protection that limits the time faults flow not the current. A RCD is no substitute for good design and construction.
PS for purists I know (supplementary and direct /indirect contact are now dropped from 17th edition but i cant for life of me remember new definitions)
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Posted By Robert k Mitchell
i think Jonf is saying that if a earth isn't present or going FROM a rcd socket ie phase and neutral only to say a garden appliance. how would the RCD trigger if there isn't a earth wire going back to RCD an how would the live know it was being earthed suddenly if i picked up the cable which was bare since the earth path would be not be back through the RCD or consumer. Would it be correct to say that the power drawn through my body to ground would eventually trip the RCD?
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Posted By Robert k Mitchell
Recently i have seen a Glen Bathroom heater fitted with a inline RCD 30MA as additional protection. This also didn't have a Earth path back to the RCD. this was in a domestic house and i also did wonder whether this would actually work without the earth not connected at RCD. thankfully it works very well and hopefully will never have to be tested in a real life fault scenario. i have stayed at my mates house and he says its ok and not to worry and carry on reading and learning my BS7671
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Posted By Ian Gardner
Robert, you are quite correct in your first post.
An RCD 'compares' the current flowing 'out of' the live conductor with the current flowing 'into' the neutral conductor. Under normal operating circumstances, these should be pretty much equal.
The RCD will trip if it detects an inequality (above a certain preset threshold) between the live and neutral currents. For most applications (ie. plug-in RCD adaptors & similar) this will be 30mA. This is the 'residual current' that gives the device it's name.
Where this residual current goes depends on the fault in question. Yes, it could be flowing into an earth conductor on a Class I appliance. Just as likely, it could be finding a path to earth via an unlucky human being, who has just chopped their garden extension lead with their Class II lawnmower.
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