100ma RCD mainswitch question

[Dunx]

Hi

I have a CU to upgrade, its a large Wylex metal clad, loads of room and slack on cabling and dinrail mounted mcbs.Main switch is a 100ma 80A. I was thinking of leaving all as is and replacing the MCB's with RCBO's. Any reason I can't leave the 100ma in place?!

Ta

Dunx

 

[Andy™]

unless its time delay, you have no discrimination

 

[Doc Hudson]

Earthing type? Do the supply tails require RCD protection? Why bother putting in RCBO's? if you are leaving the 100ma in place? Are you not worried about discrimination? Is this a college question?

Doc H.

 

[Dunx]

No not a college question, RCD's required as it is a domestic upgrade. Its not occurred before, I couldn't see why discrimination would be an issue as the RCBOs would be 30ma downstream. It was just me being lazy! I could simply change the 100ma for a 30ma but that would leave all circuits on one RCD so i thought of using RCBO's..... Can't remember what the earthing was but can't see the significance anyway?

 

[Andy™]

maybe you need to go back to college to learn how an RCD works, and the impact the earthing type has on your choices

 

[Doc Hudson]

No not a college question, RCD's required as it is a domestic upgrade. Its not occurred before, I couldn't see why discrimination would be an issue as the RCBOs would be 30ma downstream. It was just me being lazy! I could simply change the 100ma for a 30ma but that would leave all circuits on one RCD so i thought of using RCBO's..... Can't remember what the earthing was but can't see the significance anyway?

Consider a fault of 0.5amp (500ma) magnitude occurring on anyone of the RCBO protected circuits. It would be pot luck if the 30ma RCBO or 100ma RCD tripped first. in which case there is no point in fitting RCBO's as the only discrimination you would get is on faults of less than 50ma. If the earthing were TT that may be why the 100ma was at the front end, but the metal enclosure and tails would not be protected. These are all basic checks that would be undertaken whilst assessing the work that was required and may help decide whether a front end RCD is needed on the tails or not. This is why I assumed it was a college question?

Doc H.

 

[Dunx]

Had a bad week Andy? Either explain or go back to college for 'people skills'

 

[Andy™]

Had a bad week Andy? Either explain or go back to college for 'people skills'

yes, with people claiming to be electricians, yet not knowing the basics

 

[Doc Hudson]

Had a bad week Andy? Either explain or go back to college for 'people skills'

I suspect Andy is more worried that this is not to be a college question and that competent persons lack basic knowledge about how protective devices work or that different earthing types have different requirement for protective devices. To some degree I can see his point. Though I have tried to illustrate the points you should have considered whilst on site. Hopefully you understand now?

Doc H.

 

[Dunx]

Doh, should have said its either TNS or TNCS, I very rarely see TT!

 

[SLIPSHOD & SLAPDASH]

If its not TT (and it shouldn't be in a metalclad board if it is) then swap out the 100ma RCD for a100A switch

 

[steptoe]

Doh, should have said its either TNS or TNCS, I very rarely see TT!

maybe you have now spotted the lesser known superior TT system in its natural habitat,

are you really an electrician?

or just another professional DIYer?

 

[NozSpark]

If its not TT (and it shouldn't be in a metalclad board if it is) then swap out the 100ma RCD for a100A switch

Jeez Slips,

Don't tell him that, he might just do it with no regard as to what the final circuits supply:O

 

[Andy™]

or just another unprofessional DIYer?

Fixed the quote for you.

 

[PC Electrics]

HiI have a CU to upgrade, its a large Wylex metal clad, loads of room and slack on cabling and dinrail mounted mcbs.Main switch is a 100ma 80A. I was thinking of leaving all as is and replacing the MCB's with RCBO's. Any reason I can't leave the 100ma in place?!Ta

Dunx

Yes. The most obvious and glaring reason is that you will have no discrimination between the RCD side of the RCBOs and the RCD main switch !!

As Manator has explained, any earth fault anywhere in the system could trip the main switch before its relevant RCBO

 

[Dunx]

Gentlemen I am sorry I have troubled you, I shall not be back.

 

[kme]

(waves forlornly)

"Bye"

 

[Doc Hudson]

Gentlemen I am sorry I have troubled you, I shall not be back.

Consider a fault of 0.5amp (500ma) magnitude occurring on anyone of the RCBO protected circuits. It would be pot luck if the 30ma RCBO or 100ma RCD tripped first. in which case there is no point in fitting RCBO's as the only discrimination you would get is on faults of less than 50ma. If the earthing were TT that may be why the 100ma was at the front end, but the metal enclosure and tails would not be protected. These are all basic checks that would be undertaken whilst assessing the work that was required and may help decide whether a front end RCD is needed on the tails or not. This is why I assumed it was a college question?Doc H.

If you actually read all of the post on this thread, various members, myself included, have explained in relatively easy to understand language the underpinning electrical principals and design considerations that are glaringly wrong with your initial suggestion. I am genuinely worried by your last comment that you still don't actually understand the concept that you have got wrong with your initial suggested design. It is hard to say to someone you have got something completely wrong and often forum members try to nudge people to think in the correct way to help them understand how to solve a problem. I do hope I am wrong, but I am honestly not sure if you have grasped the answer from the posts given, but suffice to say, your first post is completely wrong.

Doc H.

 

[Jono Pashley]

Ta tar m8

 

[steptoe]

maybe he didnt have the required 2330/2360 ?

if you dont have that your never going to be a spark,

and thats a fact! along with the fact that the JIB are dying and clutching at as many straws as they can!