Rcd's in series..

[Chr!s]

seriously?!thats madness,

so you are testing the effectiveness of a device that measures an leakage to earth and you dont use the earth to test it with?

what about ELCBs?

you may as well just short out L load with N supply using a bit of wire,

IMHO that test isnt worth much more than pushing the test button,

Its testing the tripping mechanism and the disconnection times at a given imbalance. It tests the RCD.

Ill give you a scenario that many advise yourself included. The RCD dosent trip when testing, " have you disconnected the load from the RCD". So the connected load stops the RCD from working we disconnect it, so we can test. The RCD may not trip due to the connected loads when in normal operation but were happy to test it that way.

In effect your testing the Tripping mechanism only, not the connected system.

 

[steptoe]

something new to be learned every day,

Im still not impressed with that method,

the whole purpose of an RCD is in my mind to detect a leakage to earth, verified by the imbalance between L-N,

testing it with this method is not verifying the earth path will be good enough to trip the RCD.

still, if GN3 is happy, then why not,

cant see myself using that method though.

This is not an Earth test. Thats a seperate Ra Test for TT or Ze for TN

but surely it should be taking into account the earth path for tripping fast enough under fault conditions,

your Zs could show 334 ohms , your method of testing doesnt prove the RCD will trip fast enough under those conditions, unless the fault is to N,

which will most probably be on the load side and therefore the RCD wont trip at all, as there is no imbalance.

 

[Sidewinder]

Sorry PaulThe other option we have currently is to turn the area of the premises being used into a Group 2 Medical Location, see section 710, BS7671:2008 Amd 1:2011, the cost would be exorbitant, and the complexity of doing this in the environment would be highly disruptive.

Group 2, or Clinical Risk Cat 4,5. Has a risk assessment really shown this to be the case?

The environment is specifically listed in 710 as a Group 2.

I disagree with this, hence my approach.

Also there are other recognised standards that put the locations in a lower risk category.

---------- Post Auto-Merged at 19:34 ---------- Previous post was made at 19:31 ----------

Steps, this is called strapping the RCD, you will find reference to it in later versions of Fluke and Megger manuals. Your testing the RCD, not the connected system.This is also the method to use where a high Ra may be present.

You may find this in the new GN3, as they are aware of the 50V requirement placed on the test equipment manufactures. If you have a Ra greater than 333 ohms it will not test, yet a Ra 0f 333 ohms is allowed by BS7671, so the only way to test is to strap. The Manufacturers of the RCDs are also happy with this method.

I'm not getting into the debate on this, however, I am also currently writing an email to a friend of mine on the GN3 committee, I'll ask if this method is to be recognised.

The 50V requirement is with regard to the requirement of 7671 that touch voltages must be less than 50V.

Can you reference the manufacturers data that states this is acceptable please?

 

[steptoe]

Some of the Siemens 5SM range will only do a trip test for disconnection times if you strap it to do the test as above, spoke to their technical and depending on the install they can be a bit quirky with a meter trip test....@dad - didnt you know this is a standard test :slap

NO.!

I still think its a rubbish idea. :|

Im trying to work out how this is even relevant to what could be considered (in domestic conditions anyway) an RCDs primary function, which would normally be to detect a fault leakage to earth.

 

[green-hornet]

I remember Special Location putting me right on this subject of testing RCD's many years ago, and to this day I still disconnect loads to carry out testing.

 

[steptoe]

I think this is a backward step,

bit like when we didnt have D-Lock meters and we used to strap over the L at the RCD to take a Zs,

 

[green-hornet]

I still for my own pleasure test at the extreme of a radial or middle of a ring, just to satisfy my own interest and I am always surprised that each connected circuit reacts at a different time, but all are well within the times required.

 

[steptoe]

different Zs will give a slightly different trip time G-H

this is why Im confused as to why/how you can test an RCD by omitting the earth path completely

 

[Chr!s]

The environment is specifically listed in 710 as a Group 2.I disagree with this, hence my approach.

Also there are other recognised standards that put the locations in a lower risk category.

Well non of the standard's are statutory, use a mixture of HTM, MEIGaN and BS7671, though im still not convinced 3 RCD'S in series is the answer.

---------- Post Auto-Merged at 19:34 ---------- Previous post was made at 19:31 ----------

I'm not getting into the debate on this, however, I am also currently writing an email to a friend of mine on the GN3 committee, I'll ask if this method is to be recognised.

The 50V requirement is with regard to the requirement of 7671 that touch voltages must be less than 50V.

Can you reference the manufacturers data that states this is acceptable please?

Well the Gentleman who's job it is to update GN3 is aware of the scenario, we will wait and see. Paul, touch voltage's may be much higher than 50V, BS7671 only considers touch voltage where supplementary bonding is required and other special locations. The 50V limit comes from the manufacturing standard for the test equipment.

 

[steptoe]

is the 50v limit even referenced in 7671 anymore?

 

[Sidewinder]

I think that you will find that the 50 volts is "related" to 411.5.3, which will relate back to the IEC std for both installs & the instruments.

I am just curious as to where you can state categorically that all of the GN3 committee are aware of your points, even though I am not sure that you have explained them sufficiently here?

Oh, & I DON'T install 3 series RCD's, I install 2 series RCD's, there just may be another upstream device already installed.

Until I can get some more details from the IET then I will continue as I am.

The test method you refer to in the Fluke manual is for IT systems, these are prohibited from general use by BS7671 in the UK, to such an extent they are now moved to an Appendix.

I don't have time to reference the regs now as I have some work to finish.

 

[Chr!s]

I think that you will find that the 50 volts is "related" to 411.5.3, which will relate back to the IEC std for both installs & the instruments.No, i think you will find its not, RCD'S do not limit touch voltage. I have the Email from fluke regarding the manufacturing standard, ill see if i can dig it out.

I am just curious as to where you can state categorically that all of the GN3 committee are aware of your points, even though I am not sure that you have explained them sufficiently here?

Well, ive spoken to two of them regarding the issue and have emails stating possible clarification in the amended GN3, as i have regarding testing of bonding conductors.

Oh, & I DON'T install 3 series RCD's, I install 2 series RCD's, there just may be another upstream device already installed.

Until I can get some more details from the IET then I will continue as I am.

Well if its group 2, you will need IPS(IT), UPS, Specific Earthing arrangements, and service pendants, all of which should be achievable without to much cost, certainly where there's a risk of life, is there a risk of microshock?

The test method you refer to in the Fluke manual is for IT systems, these are prohibited from general use by BS7671 in the UK, to such an extent they are now moved to an Appendix.

Isn't it an IT system required for group 2?

Its not just for IT systems, they have it in their standard manual, just cant find it.

I don't have time to reference the regs now as I have some work to finish.

Work at this time of night, do you sleep lol

 

[steptoe]

RCD'S do not limit touch voltage

correct,

Zs limits touch voltage,

and that is why its so important (imho) to ensure you have a good enough earth path when testing an RCD for it to trip fast enough,

using the method described here would not do so.

 

[Sidewinder]

Chris,

You are getting somewhat confused here.

The reason the testers limit testing is when the touch voltage under test conditions can exceed that as described in the reg I quote, I did NOT EVER state that RCD's limit touch voltage.

The work involved in converting the locations to a Group 2 standard would be prohibitive, and extremely expensive.

I do not believe that 710 was ever envisaged for this work.

There are applied parts, the risk of microshock is present, but limited.

IT systems are required for Group 2 locations, however, I don't class the installs I am doing as Group 2 even though 710 does.

To separate and convert the existing install to IT would be a huge upheaval that would not be workable.

Still have a few things to finish, only got out of the workshop at 19:00, but, enough for tonight, back at a suppliers for 7:30 then to a couple of others, then back to the workshop all day, and in and out of the office.

Don't have the option of nice hours, have to do the work when it is there to keep the clients happy, especially once I commit to a medical job, the time-scale cannot change once confirmed.

 

[Chr!s]

correct,Zs limits touch voltage,

and that is why its so important (imho) to ensure you have a good enough earth path when testing an RCD for it to trip fast enough,

using the method described here would not do so.

Incorrect R2 determines touch voltage

 

[Sidewinder]

Incorrect R2 determines touch voltage

Almost

It is the "earth" side of the Zs path that limits touch voltage, not "just" the R2 of the circuit, it will be this combined with any other parallel paths and the external earth fault path impedance, along with any other earth paths in parallel with this.

Also, it will be the impedance rather than the resistance that works to limit the touch voltage.

 

[Chr!s]

Chris,You are getting somewhat confused here.

The reason the testers limit testing is when the touch voltage under test conditions can exceed that as described in the reg I quote, I did NOT EVER state that RCD's limit touch voltage.

I think that you will find that the 50 volts is "related" to 411.5.3, which will relate back to the IEC std for both installs & the instruments.

We were talking about touch voltage, then you posted the above, this Reg does not place a limit on touch voltage as such, just ensures voltages over 50V are disconnected.The requirement will come from an IEC, probably more to do with the acceptance of 50V being a safe touch voltage.

The work involved in converting the locations to a Group 2 standard would be prohibitive, and extremely expensive.

I do not believe that 710 was ever envisaged for this work.

There are applied parts, the risk of microshock is present, but limited.

IT systems are required for Group 2 locations, however, I don't class the installs I am doing as Group 2 even though 710 does.

To separate and convert the existing install to IT would be a huge upheaval that would not be workable.

A small ups for the required circuits, IPS for the sockets, the earthing shouldn't be to much of a issue, pendants, well you could get around them, surly if there is a risk, then the above is achievable without too much cost?

Still have a few things to finish, only got out of the workshop at 19:00, but, enough for tonight, back at a suppliers for 7:30 then to a couple of others, then back to the workshop all day, and in and out of the office.

Don't have the option of nice hours, have to do the work when it is there to keep the clients happy, especially ]once I commit to a medical job, the time-scale cannot change once confirmed.

Very Interesting

 

[steptoe]

AlmostIt is the "earth" side of the Zs path that limits touch voltage, not "just" the R2 of the circuit, it will be this combined with any other parallel paths and the external earth fault path impedance, along with any other earth paths in parallel with this.

Also, it will be the impedance rather than the resistance that works to limit the touch voltage.

^^

agreed, touch voltage can only be limited by the resistance to star point,

R2 merely proves the existence of continuity back to MET

 

[Chr!s]

Almost It is the "earth" side of the Zs path that limits touch voltage, not "just" the R2 of the circuit, it will be this combined with any other parallel paths and the external earth fault path impedance, along with any other earth paths in parallel with this.

Also, it will be the impedance rather than the resistance that works to limit the touch voltage.

Ah, i said determines, not limits

 

[steptoe]

how does R2 determine touch voltage then?