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# RCD protection maths?

Discuss RCD protection maths? in the Electrical Forum area at ElectriciansForums.net

#### spud1

##### Regular EF Member
Hi all,

My colleague and I were having a discussion today about RCD protection, as you do, and the reasoning behind why it is stipulated that 30mA should be the rating of an RCD for additional protection. Now we all know the rules regarding the maximum trip times for the various tests, but I just wanted to understand it more and ask how the maths works? Is an earth fault of 30mA at the various maximum trip times a significant figure in terms of survivable electric shock? Im not sure I quite fully 'get' how the 40msx1 and the 200/300msx5 rule works mathematically? Does it bear any relation to the 0.4secs disconnection time that we are supposed to achieve for circuits <32A and or distribution circuits in TN systems? and the 0.2secs for TT...?

Hope that makes sense?

Thanks

#### telectrix

##### Scouser and Proud of It
Respected Member
30mA for 300mS is considered to be a surviveable through the body current. some say 10mA would be even safer, though with today's electronic gear everywhere, that would probably cause nuisance trips. the 5x figure assumes that a 150mA current needs interrupting quicker.

#### spud1

##### Regular EF Member
Thanks Telectrix. So does the 300mS bear any relation to our 0.4sec (400mS) maximum disconnection time? And why are the older 4293 RCDs afforded a 200mS threshold?

#### Shoei

##### Regular EF Member
The BEAMA RCD Handbook has some information about the physiology of electric shock protection, pages 8 and 9.

Archy - snap!

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#### telectrix

##### Scouser and Proud of It
Respected Member
Thanks Telectrix. So does the 300mS bear any relation to our 0.4sec (400mS) maximum disconnection time? And why are the older 4293 RCDs afforded a 200mS threshold?
no relation. the 0.4 secs.is the disconnect time for fault protection, whereas the RCD is for additional protection.

#### gazdkw82

##### Trainee
Trainee Access
Between 10-40ma, cardiac arrest is possible.

#### spud1

##### Regular EF Member
Beama guide answers my questions thanks guys ! Thanks too tel for clarification on disconnection time.

Love this forum!

#### telectrix

##### Scouser and Proud of It
Respected Member
Beama guide answers my questions thanks guys ! Thanks too tel for clarification on disconnection time.

Love this forum!
just send a magnum or 2 of scrumpy. .

#### spud1

##### Regular EF Member
Rite eway me old mucker! (Thats actually how I talk...)

Respected Member
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