High Zs on cct

[pevvers]

Ha Ha, you're only right - I think I was getting my 200ms and my 300mA mixed up - I did have the kids screaming around me when I typed that before

I'd guessed that, but just wanted to be pedantic!

I understand the touch voltage calcs that you went to great lengths to explain

I had no doubts you did, but if one of the newer sparks was to read this then it is always helpful, as the amount of people I have spoken to that don't know that all the 3871/60898 values are derived from Ohms law is incredible!

My point was that if you are using a 300mA up front for earth fault protection, then you don't get 1667 as a Zs, just because you have 30mA RCDs installed for additional protection against electric shock.
I'd work off the 300mA Zs figure - ie. 167 ohms, just like I'd work off the Zs figures for the MCBs in a TN system.

If it was a TT system, then I would be attempting to get all readings below 100 ohms no matter what the front end RCD is. If it was a TN system, then I would completely disregard all maximum Zs's relating to RCD's and just work to the max Zs's from MCB's as you would.

Does that make sense?

A little!

 

[pevvers]

Unfortunately we cannot obtain a B type RCBO as schneider do not make one to fit their tp+n db's.

Schneider as in Merlin Gerin Schneider? Are you completely sure you cannot get a B type?

Also, when you say that a B type RCBO won't fit their enclosure, then surely they're the same size as a C type?!

 

[Mr Mark Sparks]

Schneider as in Merlin Gerin Schneider? Are you completely sure you cannot get a B type?

i tried to get a b type merlin/schneider and was told not made also


nick

going down a rough road a neweys own will fit and they do a b type but i doubt the hosp will let it be fitted

 

[WayneL]

I had no doubts you did, but if one of the newer sparks was to read this then it is always helpful, as the amount of people I have spoken to that don't know that all the 3871/60898 values are derived from Ohms law is incredible!

Point taken, mate

On that note - you'd be the man to answer this, then.
I'm sure I've asked something about this before, but I'm not sure if it's been explained properly.
If you can spell it out, it might help others as well.

Why, in table 41.5, are the max Zs values for a 30mA Rcd all 1667 ohms, except for the value given in the third column (230v < Uo <= 400v), where it is 1533 ohms?

 

[Guest123]

Point taken, mate

On that note - you'd be the man to answer this, then.
I'm sure I've asked something about this before, but I'm not sure if it's been explained properly.
If you can spell it out, it might help others as well.

Why, in table 41.5, are the max Zs values for a 30mA Rcd all 1667 ohms, except for the value given in the third column (230v < Uo <= 400v), where it is 1533 ohms?

You didn't like the answer I gave last time then Wayne????

 

[WayneL]

You didn't like the answer I gave last time then Wayne????

What was it, mate?

With pevvers bringing the table and values up, it reminded me of the question, but I can't remember what the answer was

I had a flick through my previous posts, but couldn't find the thread that it was in.

 

[Guest123]

http://www.electriciansforums.net/e...tions/4810-max-zs-puzzler.html?highlight=1533

It's all to do with the fault current needed for a 30mA RCD to operate on the fast trip and it's relation to touch voltage which would typically be 250V (5x) @ 150mA.

 

[WayneL]

It's alright Len, I'd just found the thread from last year myself - gonna have a read through it again - see if it makes sense

 

[Guest123]

It's alright Len, I'd just found the thread from last year myself - gonna have a read through it again - see if it makes sense

I'm sure it will to you now.

 

[Mr Mark Sparks]

http://www.electriciansforums.net/e...tions/4810-max-zs-puzzler.html?highlight=1533

It's all to do with the fault current needed for a 30mA RCD to operate on the fast trip and it's relation to touch voltage which would typically be 250V (5x) @ 150mA.

cheers

 

[WayneL]

I'm sure it will to you now.

Ha ha, I thought that as well - with me knowing a bit more now, but I've just re-read the whole thread and I'm still struggling to get my head around it

I get the touch voltage bit, It's the fact that it's lower than the other figures in that one column only??

And then in the next colum (>400v) it's back to 1667

 

[Guest123]

Right....a 30mA RCD will operate @ 1 x IdeltaN (30mA) before the touch voltage of 50V is reached as long as the maximum EFLI is no more than the product of the following sum....

50/0.03 = 1667ohms maximum.

In order for it to operate instantaneously i.e @ 5 x IdeltaN (150mA) the touch voltage that could be obtained would be 250V hence the value of < 230 but >400

The sum then becomes....

230/0.15 - 1533ohms maximum.


@ 30mA fault voltage would only reach 50V, @ 150mA fault voltage would reach 250V. 1x and 5x respectively.


Any good???

 

[Deleted member 9648]

Hi All, the job was designed under the 16th edition and so the figure of 1.20 was from the 16th ed regs .

I know we are now installing to the 17th , but on these bigger jobs as long as the design was signed off under the 16th edition we test and install to the 16th .Its great working with big engineering consultancies .

Unfortunately we cannot obtain a B type RCBO as schneider do not make one to fit their tp+n db's.

I am going to try retesting the circuit with our high accuracy megger ltw425 and hopefully will come under the reading. Failing that i am going to insist all sockets are removed and all connections checked to try and bring it down. The circuit in question is a cleaners socket circuit in a hospital corridor so a 16A is out of the question unfortunateley .


Nick[/QUOTE

The pont some of you are missing is that yes, while a 30ma rcd is used for supplementary protection against electric shock it can also be used for earth fault protection. Changing the mcb type to one which will be within the max Zs value is just to ensure that the protective device will operate within the required disconnection time in the event of an earth fault.What is the point of changing the mcb when the rcd already ensures the required disconnection time is met?...change an mcb type....add rcd protection...the end result is the same....Compliance with bs 7671...... It is just the type of protective device used to comply which changes.

When designing and installing a new circuit it would be inexcusable to end up with a higher Zs than the mcb protecting the circuit .But we dont always have the luxury of designing the circuit, more often than not we are altering and extending existing circuits designed by others, often many years ago, and this is where these slightly high zs issues crop up.

Sparkydude...The only purpose of a Zs test is to ensure that required disconnection time is met for the protective device in use.Other tests being satisfactory your circuit protected by an rcbo complies with bs 7671 and meets the required disconnection time. In my opinion you are wasting your time trying to gain a fraction of an ohm for no improvement in the disconnection time which is already going to be satisfactory.

 

[Guest123]

In an existing situation I agree with you wirepuller, I was referring to a new circuit just installed that ordinarily would not require RCD protection but as the max Zs was exceeded for whatever reason, now would need it.

This I have to say is bye the bye as it's a socket outlet for use by ordinary persons it needs RCD protection anyway so the Zs for the MCB part is negated.

 

[WayneL]

Right....a 30mA RCD will operate @ 1 x IdeltaN (30mA) before the touch voltage of 50V is reached as long as the maximum EFLI is no more than the product of the following sum....

50/0.03 = 1667ohms maximum.

In order for it to operate instantaneously i.e @ 5 x IdeltaN (150mA) the touch voltage that could be obtained would be 250V hence the value of < 230 but >400

The sum then becomes....

230/0.15 - 1533ohms maximum.


@ 30mA fault voltage would only reach 50V, @ 150mA fault voltage would reach 250V. 1x and 5x respectively.


Any good???

No, sorrry

Everything you are saying makes sense, but it still doesn't explain why it drops in that one column only.

The 5x IdeltaN doesn't make sense because that only relates to 30mA Rcds, but column 3 values change for all Rcds...30mA, 100mA, 300mA & 500mA....then change back again for column 4.

Let's take our formula:

Ra x I delta n = 50v .....This is one of the requirements that have to be met if RCD used for earth fault protection.

so...

Ra = 50/I delta n

For a 30mA RCD

Ra = 50/.03 = 1667 ohms

Now the figures used above remain constant throughout the 4 columns in table 41.5

i.e. touch voltage 50v, current 30mA .........coz that's the equation they have to satisfy

the only thing that changes is the resistance figure in column 3


51v to120v...............1667

121v to 230v..............1667

231v to 400v..............1533

401v & higher.............1667

So why, at this nominal voltage, does it alter?

Why does it alter back again in the next column?

And there is one more question I'd like to ask.

As our regs don't cover supply distribution, where are we likely to encounter Uo bigger than 230v (like column 3 & 4 refer to)?
Would this be on private supply systems, as opposed to public?