Question

[Poshspice1986]

How do you work out what type of RCD you need.
But more importantly why? I understand how it works. the benefits but I don't understand why.

 

[telectrix]

for most applications a 30mA RCD . are you talking domestic?

 

[DoBBY]

You don't work it out, there is regulations as to what rating RCD is needed for certain installations in BS7671

 

[dim_bulb]

30 mA isnt enough to kill a person the one up from that is the 100 mA which usually is used as a main isolator on a TT system etc
or is your question regarding tripping times?

 

[Deleted member 26818]

There are many instances where working out is required.
Where earth electrodes are used for instance.
Circuits where values of Zs exceed those required for operation of overcurrent protection devices for another instance.

 

[IQ Electrical]

How do you work out what type of RCD you need.
But more importantly why? I understand how it works. the benefits but I don't understand why.

The guide below is an Irish publication and from 2005 but for the applications and characteristics of RCD/RCBO's, it's all still pretty relevant and clearly explained.

http://www.etci.ie/docs/ET214-2005.pdf

 

[Engineer54]

30 mA isnt enough to kill a person the one up from that is the 100 mA which usually is used as a main isolator on a TT system etc
or is your question regarding tripping times?

Not correct, 30mA can kill you. 30mA protection was chosen as the norm for two reasons, one being that, that is the recognised valve for the threshold of heart fibrillation. In normal conditions/situations 30mA will indeed be a good safety protection value, but 10 mA would be closer to your above posts comment. The other is purely based on nuisance tripping occurrences. Can you imagine having 10mA RCD devices everywhere in our CUs, ....it'll be a bloody nightmare!! haha!!!

 

[Poshspice1986]

yes but why is there 500mA and 300mA and 100mA and 30mA and how do you know which to design for? There must be some equation and cases where smaller disconnection times are needed.

 

[telectrix]

it's not so much disconnection times as the cvurrent value seen by the RCD to cause it to disconnect. regs are specific on the types and ratings of RCDs in different situations.

 

[Deleted member 26818]

30mA RCDs are used for protection of persons.
100mA and higher are used for protection of property.
Choosing the higher ranges will depend on the location, and the Zs of the circuit being protected.

 

[IQ Electrical]

yes but why is there 500mA and 300mA and 100mA and 30mA and how do you know which to design for? There must be some equation and cases where smaller disconnection times are needed.

So you didn't read the information in the link that I posted then?

It's all there.

 

[Deleted member 26818]

Not quite IQ, there doesn't appear to be any mention of 500mA RCDs.

 

[IQ Electrical]

30mA RCDs are used for protection of persons.
100mA and higher are used for protection of property.
Choosing the higher ranges will depend on the location, and the Zs of the circuit being protected.

You're right, who posted that rubbish?

 

[SPARTYKUS]

poshspice1986

It's all in the big red book!

 

[Deleted member 26818]

You're right, who posted that rubbish?

All that RA x IΔn < 50V, who needs it?
Way too complicated.