High Zs on cct

[bugsy]

Nice thread, took a long time to get an answer for swapping for type b though . MG have never made a C60H type B rcbo dunno why ,

 

[pennywise]

Because most of their boards are used in commercial environments and so demand isn't great enough to warrant a B type maybe?

 

[Guest123]

No, sorrry

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

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

Because the touch voltage theoretically will be 250V (5 x 50V) which exceeds 230V but not 400V.
But as we dont have a 250V voltage the sum is calculated using our 230V which gives us the resistance of 1533ohms.

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?

Obviously there is an allowance for our supply voltage as it is never bang on 230V hence the allowed tolerances.

 

[WayneL]

Hi Len,

I've just altered my other post.

What I said was that I don't see how the 5x IdeltaN comes into it as this only applies to 30mA RCDs, yet the values in column 3 change for all RCDs and then change back again

 

[WayneL]

Obviously there is an allowance for our supply voltage as it is never bang on 230V hence the allowed tolerances.

Yeh, but it's not going as high as 400v


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I'm sorry if I'm getting annoying with this now, but I'm one of those irritating b""""""s who won't let something go until I understand it

 

[Guest123]

I see what your saying....because the instantaneous trip is only applicable to a 30mA RCD.

If you do the calc though....230/0.5 (5 x IdeltaN for a 100mA RCD) = 460ohms etc etc.

It appears that whole column is adjusted for an instantaneous trip with the higher fault voltage when in fact only a 30mA is required to provide it.

 

[Guest123]

I'm sorry if I'm getting annoying with this now, but I'm one of those irritating b""""""s who won't let something go until I understand it

No annoying at all mate.....I like a challenge.

 

[WayneL]

No annoying at all mate.....I like a challenge.

I think I've worked it out, Len.

We're talking about disconnection times on a TT system.

We know that 50 / .03 =1667 (30mA) for touch voltage compliance

In table 41.1, column 2, nominal voltge up to 230v, is our standard disconnection times - 0.4s TN and 0.2 s TT.

Now the time/current characteristics table for RCDs, on pg 243 of the regs, tells us we can meet those disconnection times with a residual current of about 2x IdeltaN - (roughly 60mA)

Remember, were talking about fault protection now, not additional protection.

So, 120v (the lowest in the range) / 1667 = 0.071 A - this works

The minute you move to the next column, your disconnection time drops to 0.07 sec, so, we'd need a residual current of 5x IdeltaN to meet this (pg243 again), yet the lowest voltage in the range is only 231v, so...

231v / 1667 = 0.138A.......we need 150mA to make disconnection - no good

So, you already gave us the calculation - 230 / 1533 = 0.150 A - this works

Column 4, the lowest voltage in the range is 400v, disconnection time 0.04 - again we need 5x I

400v / 1667 = 0.239 A - this works, no problem

Thank god for that, I'll be able to sleep now

 

[Guest123]

Happy days.

 

[Mr Mark Sparks]

Nice thread, took a long time to get an answer for swapping for type b though . MG have never made a C60H type B rcbo dunno why ,

my 1st post on page 1 mentioned this and also the chance of contacting the manufacturer to see if he would allow a higher zs
the thread just went a little off piste

 

[Deleted member 9648]

my 1st post on page 1 mentioned this and also the chance of contacting the manufacturer to see if he would allow a higher zs
the thread just went a little off piste

The circuit is protected by an rcbo.....the max Zs for the mcb part of the rcbo is rendered irrelevant by the rcd part of the rcbo which will ensure the disconnection time is achieved at a Zs of up to 1667 ohms.

 

[Mr Mark Sparks]

The circuit is protected by an rcbo.....the max Zs for the mcb part of the rcbo is rendered irrelevant by the rcd part of the rcbo which will ensure the disconnection time is achieved at a Zs of up to 1667 ohms.

so with the danger of going over this again

you dont need to worry what the zs is if the circuit is rcd protected

i am not starting an arguement just playing devils!

 

[Deleted member 9648]

Of course the zs is relevant....but any problems with a very high zs on a TN system should be picked up and rectified before the zs is even measured...high ze or R1R2 for instance.
In the case discussed here the measured zs is only a tiny fraction of an ohm above the max zs for the mcb to meet the required disconnection time.If the mcb was the only means of meeting the disconnection time it would need to be changed,but in this instance the rcd element of the rcbo ensures the disconnection time will be met up to a zs of 1667 ohms.
For the life of me I cannot see the logic of changing the mcb type to achieve a disconnection time which is already met by the existing protective device.

 

[Mr Mark Sparks]

Of course the zs is relevant....but any problems with a very high zs on a TN system should be picked up and rectified before the zs is even measured...high ze or R1R2 for instance.
In the case discussed here the measured zs is only a tiny fraction of an ohm above the max zs for the mcb to meet the required disconnection time.If the mcb was the only means of meeting the disconnection time it would need to be changed,but in this instance the rcd element of the rcbo ensures the disconnection time will be met up to a zs of 1667 ohms.
For the life of me I cannot see the logic of changing the mcb type to achieve a disconnection time which is already met by the existing protective device.

i fully agree and had this situation myself with a ring i modified and had to remove the type b mcb to fit a type c rcbo (merlin) the max recorded zs was .82 so only a little over i just noted it with the same reason you listed above

its just interesting how people interperate things

 

[devon1]

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.

I'm on a job with TN-S, RCD 30Ma, 16Amp MCB.

The engineer on site is working to the TT Zs of 1667 ohms max. (i don't think that is correct).
The measured Zs is 8 ohms on that circuit and 80 ohms on others.

The Regs give approx Zs of 2 ohms max for those cicuits on TN-S . so those circuits fail the TN-S system max Zs.
Does the fact that they have RCDs make it safe?