Some elec questions :)

hey guys, first post and just wondering on a few things.

I'll apologise now if you's get bored of this but rather than posting several different topics, one containing everything is hopefully easier

right....

With the 17th edition kicking about, everything needs RCD protecting. So what is the point in been able to purchase CU that are empty? Is it basically so people that might already have RCD/MCBs etc lyin around they can be used? Or if you buy things seprately from different places cheaper?

Can somebody explain (simple understandable terms) what is discrimination?

Can somebody explain (simple understandable terms) what is diversity?

CU with 2 RCD's, do they have to be of different AMP rating eg: 80A and 63A? And how to do you choose which to use?

Are main switches always 100A? If not/are then why? And how do you decide which to use?

If you have a service cut out of 60A, then why would your main switch be of higher rating? Surely any problems then the service fuse would blow before main switch trip??

In older installations (like mine) i've got a old looking switch that looks like an RCD before my board. Cables come out of my meter, into this switch, then to the board. Its 80A 100mA but my main switch is 100A? whats this all about? Does this count as RCD protection for my system?

sorry about that, but think that covers whats running through my head at minute might add few things as going along LOL

cheers guys, much appreciated.

Dave

Just a few clarifications:

An RCD will ALWAYS have a test button. This is how you tell them apart. Usually they have a single-wide switch on a double-wide unit. RCDs OFFER NO OVERCURRENT PROTECTION. They ONLY monitor the balance between live and neutral, therefore circuit leakage. A 63 amp RCD is rated such because this is the largest load it can safety break. It will not trip if the load exceeds 63 amps, though it may become damaged so that the trip mechanism fails (welds itself shut).

A mainswitch is JUST A SWITCH. It has NO OVERCURRENT PROTECTION. A mainswitch always has a doublewide switch on a doublewide module. I mainswitch will NEVER TRIP OF ITS OWN ACCORD. It must always be operated by a human being (or dog, cat etc)

Discrimination is basically making sure that the protective device closest to the fault operates but the one upstream doesn't. In RCD terminology you may need to use time delay models (Type S or Selective) for upstream devices.

Diversity is basically using the fact that not everything will be on and pulling power at the same time hence the total loading - if all added together - may exceed the value of the upstream protective device. The on site guide gives nominal figures for application of diversity.

RCDs don't have to be a different current rating, cheapskate manufacturers sometime fit smaller ones. That rating is only that of the RCDs contacts, the trip current is usually only a fraction of an amp.
You choose which one to use by looking at the loads and applying diversity.

Main switches are usually 100A nowadays but some were smaller. These are contact ratings and the load after diversity shouldn't exceed the rating.

The main switch is an isolator, it doesn't trip.

Your RCD may count towards RCD protection for automatic disconnection in a TT system, however only a 30mA or below trip may be used for supplementary protection against direct contact, and for protection of concealed cables.

Dave2010 said:

hey guys, first post and just wondering on a few things.

I'll apologise now if you's get bored of this but rather than posting several different topics, one containing everything is hopefully easier

right....

With the 17th edition kicking about, everything needs RCD protecting. So what is the point in been able to purchase CU that are empty? Is it basically so people that might already have RCD/MCBs etc lyin around they can be used? Or if you buy things seprately from different places cheaper?

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the box is just that, empty. Its down to the designer of the installation to fill the box with bits in such a way that it complies with the 17th edition.

Can somebody explain (simple understandable terms) what is discrimination?

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it is to do with the correct fuse blowing in the event of a fault. e.g. if you cut through the cable on your lawnmower, you really want the fuse in the plug to pop, not the main house supply fuse. This would be correct dicrimination. If the house fuse went, that would be incorrect discrimination.

Can somebody explain (simple understandable terms) what is diversity?

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in simple terms? on this forum? i doubt it
But it roughly means that you can make allowances in the circuit design and expected current demand due to the fact that not all devices will be on all the time

CU with 2 RCD's, do they have to be of different AMP rating eg: 80A and 63A? And how to do you choose which to use?

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No and that would be down to the circuit design.

Are main switches always 100A? If not/are then why? And how do you decide which to use?

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No.
because of need for different design considerations.
From circuit design

If you have a service cut out of 60A, then why would your main switch be of higher rating? Surely any problems then the service fuse would blow before main switch trip??

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Availability, cost etc.
Main switches are not trips, they are just manual switches.

In older installations (like mine) i've got a old looking switch that looks like an RCD before my board. Cables come out of my meter, into this switch, then to the board. Its 80A 100mA but my main switch is 100A? whats this all about? Does this count as RCD protection for my system?

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Sounds very much like you have a time delay RCD.
Switches and RCD's are different things, they are rated differently.
Yes and no, it is an RCD, but it is not 30mA

sorry about that, but think that covers whats running through my head at minute might add few things as going along LOL

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you think too much

cheers guys, much appreciated.

Dave

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hey guys

cheers for that very pleased. so......

Is a main switch only there for mechanical purposes incase anybody needs to work on board?

"Your RCD may count towards RCD protection for automatic disconnection in a TT system, however only a 30mA or below trip may be used for supplementary protection against direct contact, and for protection of concealed cables"

So should I have an RCD in the board to cover the circuits aswell as the one before the board?


SO does discrimination always come into play when you have 2 RCD's? And would it need to be applied in the above question?

If an RCD in a board is 80A, but your service fuse is 60A?? Would you fuse blow before the RCD trips or would the RCD detect fault quick enough to stop that?

An RCD does not detetc overloads. A fuse or MCB does.

You only need one 30mA RCD protecting one or more circuits.

A 60 amp mainfuse will blow instantly at something like 500 amps. At 100 amps it'll probably take something like half an hour to blow. These are very approximate figures.

A mainswitch is simply an isolator.

A 100mA RCD may be fitted to protect your system if you have an earth rod, since earth rods do not have a very low impedance, and therefore MCBs could struggle to meet trip times alone.

To meet the 17th edition regs all unprotected concealed cables should be protected by a 30mA device, there are exceptions to this such as when the installation is under the supervison of an instructed person however in domestic situations it is better to protect.
Cables such as Pyro and SWA don't need to be 30mA RCD protected for the purpose of them being concealed, nor do cables in earthed metal conduit.
All sockets should be on a 30mA RCD unless for a specific purpose.
A 100mA RCD may not discriminate with a 30mA device, which is where a 100mA type S RCD might be more appropriate.

Dave2010 said:

SO does discrimination always come into play when you have 2 RCD's? And would it need to be applied in the above question?

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If the RCDs are in line then discrimination would have to be considered. Many sparks install new CU's with two 30mA RCDs protecting different groups of circuits. These are independent of each other and have their own L & N feed so discrimination is not required.

Dave2010 said:

If an RCD in a board is 80A, but your service fuse is 60A?? Would you fuse blow before the RCD trips or would the RCD detect fault quick enough to stop that?

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You're getting yourself confused with the difference between an MCB and a RCD. An RCD does not detect short circuits or overcurrent faults.

Also an RCBO can be used if desired. This combines both functions.

Dave2010 said:

CU with 2 RCD's, do they have to be of different AMP rating eg: 80A and 63A? And how to do you choose which to use?

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Steve said:

RCDs OFFER NO OVERCURRENT PROTECTION. They ONLY monitor the balance between live and neutral, therefore circuit leakage. A 63 amp RCD is rated such because this is the largest load it can safety break. It will not trip if the load exceeds 63 amps, though it may become damaged so that the trip mechanism fails (welds itself shut).

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Dave2010 said:

If an RCD in a board is 80A, but your service fuse is 60A?? Would you fuse blow before the RCD trips or would the RCD detect fault quick enough to stop that?

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Dave - if you are going to ask people to answer questions you should do them the courtesy of reading their answers..

Dave - if you are going to ask people to answer questions you should do them the courtesy of reading their answers..

of course thanks

but just one more......lol

in a TT system, to meet disconnection times you might need a 100A 100mA RCD main switch? Then to comply with 17th regs would you have an RCD 30mA next to that to protect those circuits? Then would there be a 2nd RCD 30mA to protect other circuits so you are splitting them up?
Does this count as discrimination by having the 100mA and 30mA right next to each other?

cheers again guys and thanks in advance.

Dave2010 said:

in a TT system, to meet disconnection times you might need a 100A 100mA RCD main switch? Then to comply with 17th regs would you have an RCD 30mA next to that to protect those circuits? Then would there be a 2nd RCD 30mA to protect other circuits so you are splitting them up?
Does this count as discrimination by having the 100mA and 30mA right next to each other?

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OK, in a TT, if the main switch in the CU is a 30mA RCD protecting all circuits, you may omit the time delayed 100mA RCD. Because anything thats gonna trip the 100mA is gonna trip the 30mA first.

Similarly, if there are two or more 30mA RCDs, such that every circuit is covered by a 30mA RCD (typical nowadays), you may omit the 100mA RCD. RCBOs are acceptable*.

However, if there are circuits not protected by a 30mA RCD (fire alarms, fridge etc), the 100mA RCD MUST still be used. It can be fitted as the mainswitch where there is only one consumer unit.