Discrimination of RCD's

[Mike2007]

Simple 50/50 question!

Would the layout of RCDs in a system be;

Supply>10ma RCD>30ma RCD> 300ma RCD>outlets ?

Or. Supply>300ma RCD>30ma RCD>10ma RCD>outlets?

Which layout will desciminate, leaving other two RCD's closed.
That is not three devices in one line but branches,just to clarify!

 

[sparkybird]

The largest first, so 300ma to 30, to 10, same as mcb's/fuses

Why?

 

[Mike2007]

The largest first, so 300ma to 30, to 10, same as mcb's/fuses

Why?

Umm, I thought that. But was sure our tutor said the other way around??

And why do you choose a 300ma RCD for Fire Protection instead of 30ma?

 

[JohnD]

you will not get much discrimination because very often if you get a genuine fault, cracked kettle element, nail through cable etc, the current will be big enough to trip them all.

To achieve discrimination you need to have a "S" type (time delay) RCD at the upstream point, and it should be less sensitive so that it will not easily trip from the same fault, including minor background leakage from other sources.

The most sensitive and fastest RCD should always be closest to the fault.

the 300mA RCD will not give reliable personal protection from injury or death, and will be resistant to some nuisance tripping, but when the house goes up in flames and the cable insulation melts or the brigade squirts water on the CU, then it will trip and make the installation safer

A 30mA RCD is very prone to nuisance tripping, and if it protects several circuits with multiple loads, there may be enough background leakage for it to trip quite frequently

 

[Mike2007]

you will not get much discrimination because very often if you get a genuine fault, cracked kettle element, nail through cable etc, the current will be big enough to trip them all.

To achieve discrimination you need to have a "S" type (time delay) RCD at the upstream point, and it should be less sensitive so that it will not easily trip from the same fault, including minor background leakage from other sources.

The most sensitive and fastest RCD should always be closest to the fault.

the 300mA RCD will not give reliable personal protection from injury or death, and will be resistant to some nuisance tripping, but when the house goes up in flames and the cable insulation melts or the brigade squirts water on the CU, then it will trip and make the installation safer

A 30mA RCD is very prone to nuisance tripping, and if it protects several circuits with multiple loads, there may be enough background leakage for it to trip quite frequently

So don't even touch a 10ma RCD's (if knew where to get one) as it would make life impossible, yes?
How would you test RCD's in series like I have discribed, guess you would have to disconnect wiring working upstream past devices.
Adding RCD's is quick way to find out appliances that also need replacing, more expense.

 

[Spark123]

10mA devices have their place, they can be used to provide additional personal protection against fatal electric shock. The big problem with RCDs as has been pointed out is the time taken to clear a fault - if you have say 1A flowing to earth then it can take the lot out. In some cases it is unavoidable having more than one 30mA (maximum size you can have for personal protection) device in line, for example in caravan parks. One way to check an RCD without tripping the upstream device is to connect the phase and neutral of your tester to the outgoing side of the device and the earth lead on your tester to the neutral incoming side, test as normal.
Which college btw?

 

[Mike2007]

One way to check an RCD without tripping the upstream device is to connect the phase and neutral of your tester to the outgoing side of the device and the earth lead on your tester to the neutral incoming side, test as normal.

Never heard of that way before!

 

[Spark123]

Think how a TN-CS works