MCBs ADS in TT system

[Jonny72]

Quick Question. Despite being 50 I am a newbie electrical installer. Preparing for my retirement from public service. Anyways - my Question. How the hell do MCBs work on a TT system. Say Ze is 33 Ohms with a 240 supply the PFC will be just over 7 Amps. How does that trip MCBs or is there were the importance of RCDs come in. Can someone explain??

 

[nicebutdim]

In many instances ADS wont be achieved and must be satisfied by other means.

I don't mean to seem disrespectful (especially as I'm an apprentice only a few years younger than you), but this should have been covered at length when you were training.

 

[Dustydazzler]

You need to study the curve and in most cases a very low fault current won't trip an mcb , maybe a 6amp but not much else certainly not fast enough to achieve ADS

This is why TT installs had upfront RCDs Or Voltage operated earth leakage devices or what ever they were called back in the olden times

 

[davesparks]

In a TT system MCB's work on any overload or line - neutral/ line - line fault as they would in any other system.

It is only on an earth fault that they are not effective, so we use RCD's to provide fault protection.

 

[Jonny72]

In many instances ADS wont be achieved and must be satisfied by other means.

I don't mean to seem disrespectful (especially as I'm an apprentice only a few years younger than you), but this should have been covered at length when you were training.

Thanks. 28 years ago I sparked but on aircraft. Avionics to be specific. Have HNC in EE. My DEI course was basic and in no way do I see myself as a Electrician. Yea we covered the OSG in alot of detail and there was nothing about TT values apart from needs to be under 200 ohms Ze. I suppose I get credit for coming up with the question. Like you and (prob more than you)I have alot to learn

 

[James]

The best of us admit that we ARE ALWAYS LEARNING.

I have only ever met a couple of sparks who were sure they knew it all.
strangely enough I didn’t get on with either of them very well.

 

[Dustydazzler]

That is the best thing about 'electrics' its possibly the only trade where you can literally learn something new about it every week

 

[Baddegg]

Thanks. 28 years ago I sparked but on aircraft. Avionics to be specific. Have HNC in EE. My DEI course was basic and in no way do I see myself as a Electrician. Yea we covered the OSG in alot of detail and there was nothing about TT values apart from needs to be under 200 ohms Ze. I suppose I get credit for coming up with the question. Like you and (prob more than you)I have alot to learn

The best of us admit that we ARE ALWAYS LEARNING.

I have only ever met a couple of sparks who were sure they knew it all.
strangely enough I didn’t get on with either of them very well.

On that basis James we’ll get on like a house on fire

 

[nicebutdim]

That is the best thing about 'electrics' its possibly the only trade where you can literally learn something new about it every week

Every day and sometimes more than once a day

 

[westward10]

The best of us admit that we ARE ALWAYS LEARNING.

I have only ever met a couple of sparks who were sure they knew it all.
strangely enough I didn’t get on with either of them very well.

Didn't think much of you either