Voltage operated ELCB - what's wrong with them?

[batty]

Agree completely - but my customer was adamant that there was nothing wrong with it - and unfortunately he was right in this case!!!! Was hoping for a board change at a later date - but never mind!!

Just hope he doesn't live to regret it.

 

[davetheglitz]

Getting a bit fed up of people making 'random' statements, without explanationThen, people who read the posts take these points as gospel - which is how the 'fairy stories' start. (earths to outhouses lol)

So, if you don't mind QUOTE]

This was not a random statement - it was pertinent to point out that all systems have their limitations. The subject has been talked about on the forum before - and I thought my posts have been nerdy enough without going into the detail. If we took this to he Nth degree then there would be no free flow of conversation on here. However the way I look at it is as follows:-

Re RCD's and N-E faults. Consider a TT system with a N-E fault. The earth rod is now effectively shunted by a very low impedance. This means that if someone touches live there is now no mechanism for the RCD to detect this as you are now effectively connected bettween live and neutral - not good.

The only way you would know about the fault was if 30mA of current flowed through the shunted earth path.

Assume a rod with Ze of 500 ohms carring 30mA. This means the voltage across this would be I x R = 15v. Considering the shunt path to neutral - possibly 0.2 ohms - this means that 30mA would flow through the earth rod when 15/0.2 A flows through the neutral path. This is 75A - probably in most houses this never occurs.

Upshot is the RCD is effectively disabled - but the test button will still work as this is not reliant on an earth connection.

It's easy to put on a N-E fault during routine maintenance - side screws on metal light fittings especially.

Figures here illustrate the point - but if the Ze was 50 ohms the RCD would trip when 7.5A was flowing in circuit - this would manifest itself as an intermittent trip on the TT systems which seems to have no single cause. The fault has nothing to do with any one circuit as such - it's just that the load is sufficient to get 30mA of difference in the neutral current.

Thought that might kill the thread stone dead!!!!!

 

[ADS]

Thought that might kill the thread stone dead!!!!!

Looks like you were wrong about that, as well

What you are describing hardly renders it disabled, does it????

However you dress it up, the maximum current that can flow through your Neutral/Earth fault without detection will be approx 25mA (they don't all trip at 30mA)

This is the same for any earthing system, not just TT, regardless of Ze or Load - any more current than this flowing through your N/E fault and the RCD trips.

Now for the second fault - you touching the line conductor!

You are correct in the fact that the RCD won't detect the fault because of the 25mA offset current in the N/E fault until we get to approx 50mA - at which point it will trip (like I said - hardly disabled)

If a 30mA RCD will trip at well below 300mS at its rated current, I'm guessing that our (50 - 25)mA fault current will trip within 400mS.

So much for our disabled RCD

So we've had an explanation from Davetheglitz, which explains why he thought what he did - but still no explanation as to how a totally isolated circuit can trip an RCD with a N/E fault, every time - people agreeing with you doesn't count as an explanation

 

[davetheglitz]

ADS - drop the attitude and study your circuit theory!!! Draw the circuit out and think it through. Then argue coherently and politely. In the mean time reflect and grow up!! Also think about looking on the Screwfix forum - I think they are recruiting for members of your calibre!

 

[tom1]

Looks like you were wrong about that, as wellWhat you are describing hardly renders it disabled, does it????

However you dress it up, the maximum current that can flow through your Neutral/Earth fault without detection will be approx 25mA (they don't all trip at 30mA)

This is the same for any earthing system, not just TT, regardless of Ze or Load - any more current than this flowing through your N/E fault and the RCD trips.

Now for the second fault - you touching the line conductor!

You are correct in the fact that the RCD won't detect the fault because of the 25mA offset current in the N/E fault until we get to approx 50mA - at which point it will trip (like I said - hardly disabled)

If a 30mA RCD will trip in 40mS at 150mA, I'm guessing it will trip at about 120mS at our 50mA fault current - well within the 0.2 second disconnection time for TT installation.

So much for our disabled RCD

So we've had an explanation from Davetheglitz, which explains why he thought what he did - but still no explanation as to how a totally isolated circuit can trip an RCD with a N/E fault, every time - people agreeing with you doesn't count as an explanation

there was a thread on this not long back

 

[davetheglitz]

Just hope he doesn't live to regret it.

I hope he does - I might get my board change out of it!!!

 

[ADS]

ADS - drop the attitude and study your circuit theory!!! Draw the circuit out and think it through. Then argue coherently and politely. In the mean time reflect and grow up!! Also think about looking on the Screwfix forum - I think they are recruiting for members of your calibre!

There is no attitude there, it is fact.

Which part is incoherent??

What part of what I have stated are you disputing exactly??

Or is it the fact that I am right that makes you resort to this kind of posting?

Your arguments and statements hold no water, my friend!

 

[batty]

There is no attitude there, it is fact.Which part is incoherent??

What part of what I have stated are you disputing exactly??

Or is it the fact that I am right that makes you resort to this kind of posting?

Your arguments and statements hold no water, my friend!

I don't really get what you are saying. Are you really saying that an isolated circuit ie live is disconnected that if you touch neutral to earth it will not trip an RCD?

 

[tom1]

I don't really get what you are saying. Are you really saying that an isolated circuit ie live is disconnected that if you touch neutral to earth it will not trip an RCD?

just thinking that, pleanty of times i have switched off mcb cut the cable or touched the N and E and trip. once someone reset the rcd for me but did the mcb as well and i got belted

 

[davetheglitz]

There is no attitude there, it is fact.Which part is incoherent??

What part of what I have stated are you disputing exactly??

Or is it the fact that I am right that makes you resort to this kind of posting?

Your arguments and statements hold no water, my friend!

Have you dropped the attitude - No!

Have you studied your circuit theory - No!

Are you saying anything new - No!

Are you right - No!

Do my arguments hold water - yes if you read them!

Are you annoying me with your attitude and lack of politeness - YES!

When you are called out for intermiittant tripping on a TT system please reflect on this post. In the mean time please don't expect a reply if you continue in the same way.

 

[ADS]

If a 30mA RCD will trip at well below 300mS at its rated current, I'm guessing that our (50 - 25)mA fault current will trip within 400mS.

I altered this in my previous post - I'd got my maths wrong

 

[ADS]

Have you dropped the attitude - No!Have you studied your circuit theory - No!

Are you saying anything new - No!

Are you right - No!

Do my arguments hold water - yes if you read them!

Are you annoying me with your attitude and lack of politeness - YES!

When you are called out for intermiittant tripping on a TT system please reflect on this post. In the mean time please don't expect a reply if you continue in the same way.

Whatever!!

I don't really get what you are saying. Are you really saying that an isolated circuit ie live is disconnected that if you touch neutral to earth it will not trip an RCD?

If all MCBs are off in a CU and the RCDs and main switch are on, it is a physical impossibilty for the RCD to be tripped by the neutral of one of the circuits being touched to earth.

Think about what you are saying and then about how RCDs work.

Without being patronising, a circuit needs a line (supply voltage), a load and a neutral (return) for current to flow.

Without one of these - current won't flow.

So, with our line conductor isolated, there is no current flowing in the neutral of our isolated circuit, so it can't possibly flow to earth. You are connecting neutral and earth together with no current and no potential difference.

If, on the other hand, other circuits (on the same neutral bar as our isolated circuit) are energised and under load, then current could flow through the isolated circuit's neutral and down to earth.

This would create an imbalance in our RCD and it would trip - but this would only happen under these conditions.

I think this is what others on here are talking about, when they say it's happened to them.

I hope you find my answer polite and informing.

Please, if you think I am wrong, explain why - don't just tell me I'm wrong and insult me coz you don't know the answer.

Thanks

 

[batty]

Whatever!!If all MCBs are off in a CU and the RCDs and main switch are on, it is a physical impossibilty for the RCD to be tripped by the neutral of one of the circuits being touched to earth.

Think about what you are saying and then about how RCDs work.

Without being patronising, a circuit needs a line (supply voltage), a load and a neutral (return) for current to flow.

Without one of these - current won't flow.

So, with our line conductor isolated, there is no current flowing in the neutral of our isolated circuit, so it can't possibly flow to earth. You are connecting neutral and earth together with no current and no potential difference.

If, on the other hand, other circuits (on the same neutral bar as our isolated circuit) are energised and under load, then current could flow through the isolated circuit's neutral and down to earth.

This would create an imbalance in our RCD and it would trip - but this would only happen under these conditions.

I think this is what others on here are talking about, when they say it's happened to them.

I hope you find my answer polite and informing.

Please, if you think I am wrong, explain why - don't just tell me I'm wrong and insult me coz you don't know the answer.

Thanks

Sounds clear and makes sense when I get a minute I will try it at home.

 

[ADS]

Sounds clear and makes sense when I get a minute I will try it at home.

Let us know what happens

 

[davetheglitz]

ADS - I've misjudged - you do know what you are talking about - we are looking at discepancies between Line and Neutral currents to trip an RCD. Please revisit my post on TT systems with a N-E fault with this in mind and go through the maths - it was an eye opener to me when I first saw it - and it's easy to brush it under the carpet as most people are used to TN type systems where this wouldn't apply.

 

[ADS]

ADS - I've misjudged - you do know what you are talking about - we are looking at discepancies between Line and Neutral currents to trip an RCD. Please revisit my post on TT systems with a N-E fault with this in mind and go through the maths - it was an eye opener to me when I first saw it - and it's easy to brush it under the carpet as most people are used to TN type systems where this wouldn't apply.

I must admit, I had to read it three times earlier on to digest it, but I do understand what you are saying.

With a N/E fault being load-dependant and a TT system having a high impedance earth path, it is, as you say, likely to go un-noticed.

And the intermittent tripping when the load reaches a certain threshold (to send 30mA down the earth path) could cause a bit of head scratching.

This wouldn't be the case on other systems, of course, coz the current down the neutral would split more equally at the fault between neutral and earth, and therefore cause the RCD to trip under lower loads.

I think this is the gist of your post.

But I still believe that the issue is only a 25mA to 30mA difference (as this is the limit as to what will go un-noticed) - meaning that any second fault - line to earth - would need to be 30mA higher to trip the RCD - affecting disconnection times slightly, but still, in my opinion, safe.

 

[Lurch]

If all MCBs are off in a CU and the RCDs and main switch are on, it is a physical impossibilty for the RCD to be tripped by the neutral of one of the circuits being touched to earth.Think about what you are saying and then about how RCDs work.

Without being patronising, a circuit needs a line (supply voltage), a load and a neutral (return) for current to flow.

Without one of these - current won't flow.

So, with our line conductor isolated, there is no current flowing in the neutral of our isolated circuit, so it can't possibly flow to earth. You are connecting neutral and earth together with no current and no potential difference.

Absolute nonsense.

 

[davetheglitz]

I must admit, I had to read it three times earlier on to digest it, but I do understand what you are saying.With a N/E fault being load-dependant and a TT system having a high impedance earth path, it is, as you say, likely to go un-noticed.

And the intermittent tripping when the load reaches a certain threshold (to send 30mA down the earth path) could cause a bit of head scratching.

This wouldn't be the case on other systems, of course, coz the current down the neutral would split more equally at the fault between neutral and earth, and therefore cause the RCD to trip under lower loads.

I think this is the gist of your post.

But I still believe that the issue is only a 25mA to 30mA difference (as this is the limit as to what will go un-noticed) - meaning that any second fault - line to earth - would need to be 30mA higher to trip the RCD - affecting disconnection times slightly, but still, in my opinion, safe.

If you had a line to earth fault on the system it may or may not trip the RCD. The only way the RCD would trip is if there is 30mA difference between the live and neutral currents. Your additional L-E fault with the N-E fault in place effectively means its a L-N fault. However - the probability is that the MCB would trip - as the N-E fault effectively makes this a DIY TNCS system. Problem is - if the fault was on the lighting circuit the lighting cpc could be taking a substantial fault current.

 

[PC Electrics]

Sorry Lurch, but ADS' post is quite correct. It may not be written in a way that all can follow it, but it is correct.

The main point in that part of the post was that ALL MCBs are OFF. Thus, no current can flow in any part of the installation. So, since there can be no current flow, no matter how many N-E faults one creates within the installation there can be no current imbalance to trip the RCD.

 

[binky]

ALL MCBs are OFF. Thus, no current can flow in any part of the installation. So, since there can be no current flow, no matter how many N-E faults one creates within the installation there can be no current imbalance to trip the RCD.

Bang on for all MCBs off, now if you have just isolated the circuit you're working on....