Max Zs Puzzler

[WayneL]

Hi Guys,

I'm taking the 17th Edition course at the moment and I've come across a bit of a puzzler.


Table 41.5, page 50 of the Regs, gives the maximum Zs for a TT system to ensure RCD operation.


The question is this:

Why is the value for a 30mA RCD, at a voltage of between 230v and 400v, (column 3) given as

1533 ohms, when for all other voltages, both higher and lower, the max Zs is 1667 ohms?


Unfortunately I'm one of those people who won't just accept something, so I need to figure this out.

Any suggestions will be greatfully received.


Cheers,


Wayne

 

[maddfridge]

Hi Guys,

I'm taking the 17th Edition course at the moment and I've come across a bit of a puzzler.


Table 41.5, page 50 of the Regs, gives the maximum Zs for a TT system to ensure RCD operation.


The question is this:

Why is the value for a 30mA RCD, at a voltage of between 230v and 400v, (column 3) given as

1533 ohms, when for all other voltages, both higher and lower, the max Zs is 1667 ohms?


Unfortunately I'm one of those people who won't just accept something, so I need to figure this out.

Any suggestions will be greatfully received.


Cheers,


Wayne

hi there

i had not noticed that before either

As a starting point it may be lower just to meet the required disconnection time for that range of voltage but i would expect other values to change.

just done 2400 design and got a pass so some other clever person may have a defined answer

 

[Guest123]

Hey.

If Zs = 1667 ohms, then you get a touch voltage of 50V at 30mA - (1 x I delta N). At 150mA (5 x I delta N) you would have 250V touch voltage. If you wanted the RCD to operate at 5 x I delta N with the 50V safe touch voltage you would need a max Zs of < 1533 ohms.

Hope this helps

 

[Spudnik]

Good point Lenny, but then, a step on from the OP, why are we using 1667 as a max?

 

[WayneL]

Hey Lenny,

I kind of get the gist of what your saying - I think it might be a

bit advanced for me at the moment.

I'm gonna spend some time trying to get my head around the calcs.

Cheers,

Wayne

 

[Spudnik]

Wayne,

Basically 50V is regarded as the maximum touch voltage allowed through a fault before the protective device operates. IE anything that could become live through fault will not rise above 50V.

Make sense?

If not, sorry, im really not a good teacher

Get used to it. There are plenty more of these to test ya!

 

[Guest123]

Hey.

Sorry I didn't explain that one too well (baby puke issue)

For an 'instantaneous' trip in a TT system at 230V you have to assume that the trip current would have to be the 5-times figure, i.e. 150mA. Hence RA=230/0.15=1533Ohms.

At 400V this becomes RA=400/0.15=2667Ohms.

HOWEVER:

There is a requirement that the touch voltage does not go above 50V, hence the figure of 1667 is derived from maximum touch voltage of 50V and the trip current of the RCD (30mA), hence RA=50/0.03=1667Ohms.

Clearly this is lower than the RA figure for 400V above, so takes precedence over this figure.

Hope that makes sense

 

[maddfridge]

Hey.

If Zs = 1667 ohms, then you get a touch voltage of 50V at 30mA - (1 x I delta N). At 150mA (5 x I delta N) you would have 250V touch voltage. If you wanted the RCD to operate at 5 x I delta N with the 50V safe touch voltage you would need a max Zs of < 1533 ohms.

Hope this helps

hi lenny

right on mate sort of remembered that it was to do with disconection time was think aboutrcbo times not test times

cheers for clearing that up