Setting an adjustable rcd?

[wozzit99]

Hi

I'm working on a distribution system where the main s/w is a 200amp 4 pole mcb with a bolt on adjustable rcd. The rcd needs to be set to 300ma and have a time delay for discrimination purposes.

Can anyone advise to which setting on the ms adjuster I use? The ms scale goes from 0-60-150-310.

Cheers

Wozzit

 

[Knobhead]

What make and type is it?

 

[wozzit99]

Hi Tony

Make is Shneider (Merlin Gerin)
Model NS 250 MCCB with vigi bolt on rcd

 

[SPARTYKUS]

Im no expert but surely that would depend on what you were trying to discriminate against......?

 

[Knobhead]

There's plenty of information about the NS250n on the Shneider site.
you need to determine what you're discimiating against as SPARTYKUS said.

 

[wozzit99]

Discriminating against a 300ma rcd further down stream

 

[needasparks]

Strange to require an rcd incommer on 200a supply as normally a TN, what reason has it been installed for.

 

[wozzit99]

Thats the problem its a TT installation.

- - - Updated - - -

Thats the problem its a TT installation.

 

[oldtimer]

is the 0-60 for seconds delay and the 150-310 for the mA setting ?

 

[needasparks]

Thats the problem its a TT installation.

- - - Updated - - -

Thats the problem its a TT installation.

Never come across a TT so large, is it a farm etc. how have you got round the metal enclosure for incommer not being rcd protected.

 

[Engineer54]

Any chance of posting a photo of this MCCB with a close up on setting dials?? As a starting position i'd normally say around 100mS, but your set-up doesn't seem to allow that time delay.... On a standard S type RCD, tripping times are between 130 and 500 mS at x 1 sensing...

 

[wozzit99]

Oldtimer, there are two dials on the rcd part of the switch one in ma and the other in ms. The ma dial is set to 300ma although you can adjust it all the way to 10 amps if needed. The ms dial is adjustable between 0-60-150-310 ms, this is the time delay setting.

Needasparks, yes your right the installation is on a farm. I asked the dno whether they could convert the supply to TN but they said no due to the type of building the supply terminates in. In this case a metal shed in the middle of a field.

Regarding the enclosure being metal and not being rcd protected I've done all I can to limit the chances of an earth fault occuring on the case. Double insulated tails enclosed in plastic trunkin, tails not in contact with the enclosure, the switch itself is class 2 and a bit of perspex covering the metal face of the enclosure with cut outs to adjust dials. I tried getting hold of a plastic enclosure but no luck.

Engineer 54, having problems uploadind a photo of vigi unit but check out the following link
LV431536 - schneideronline.com.au this bolts on to the mccb

Wozzit

 

[Richard Burns]

Discriminating against a 300ma rcd further down stream

You are discriminating against a 300mA RCD downstream so you need to know how fast this trips, if it is a time delay one and set to say a delay of 60ms your RCD would need to be set to 150ms, assuming it is the same unit.
The units are coordinated so that a 60ms delay will trip in less than 140ms so 150ms delay would discriminate.
If the one downstream is not time delayed then yours could be set to 60.
Effectively the downstream one must trip first in order to avoid nuisance tripping.
I assume the controls are like this:

 

[wozzit99]

Cheers Richard, yes that is the exact same control panel.

Downstream RCD is a different make to the Merin above and is not time delayed.

So to sum up its a case of testing the downstream RCD, finding what its tripping time is and adjusting the vigi accordingly.

Wozz

 

[SPARTYKUS]

Again, no expert but not sure it'll work that way? Think the RCDs are wrongus wayus roundus.

I suppose it depends how quickly the up front one trips.

I think the S type 100ma ones in older TT domestic (& other) installs were put up front with about a 0.1seconds as this is long enough to be sure of discrimination...?

The way youre set up you'll be relying on the characteristics of a non time delayed RCD(if I read you right!) to be particularly slow....

And havent looked at the dials yet but if up front RCD trip in normal times (eg 35ms), not sure you have the legroom to discriminate.

Again I am no expert but find this an interesting thread and look forward to more learned input & your final resolution!


I'm wondering if the RCDs could be changed round to Mr time delay is up front

 

[Knobhead]

Who specified this unit? They would be the first to ask.

This has some interesting reading http://www.switchrite.co.za/data/merlin_gerin_din_circuit_breaker_application_guide.pdf it also gives Merlin Gerin’s help line.

 

[Richard Burns]

Cheers Richard, yes that is the exact same control panel.

Downstream RCD is a different make to the Merin above and is not time delayed.

So to sum up its a case of testing the downstream RCD, finding what its tripping time is and adjusting the vigi accordingly.

Wozz

From Spartykus post I would like to say that the arrangement I think you have is:
Supply.....Your TDRCD......non time delayed RCD......Equipment.
I.e. the time delay is upfront and closest to the supply.

You cannot just measure the trip time as conditions could be different under a fault and change the trip time. You have a different make downstream so coordination is more difficult since they will not be designed to the same characteristics. You need to know the tripping characteristics of the down stream device and your RCD and work out the value of delay that will ensure discrimination.
GN3 recommends that the trip time of an RCD does not exceed 300ms if it is not time delayed, however the RCDs you are using may have different characteristics.
For instance, I do not know the model of RCD you have but this is something similar (may be the same) that specifies the trip times for the RCD

As you can see the maximum trip times are coordinated for each setting (and are considerably less than 300ms when not time delayed).
You will need to find similar information for both your RCD and the down stream one and if you have a maximum tripping time downstream of 200ms then your RCD could be 300ms and so on.
Do some research, get on google, find the information for your devices, apply them to your situation and you are then sorted.