Hello, first time poster, long time viewer. Testing a school which on the socket circuits have BS 3871 type 3 breakers with a 30ma RCD attached. Some of the socket circuits exceed the 0.72 ohms max zs for the breaker. Should I be using the max zs as 1667ohms? I seem to remember that if max zs cannot be achieved then an RCD could be used, or have I got that wrong? Yes I am an electrician.
- Thread starter Donkey_Dave
- Start date
It's one of those don't know why, but they are type jobs. Who knows what they were thinking in the 70's! I have five circuits that exceed the max Zs, all on the socket circuits and all have the additional protection.
N
nickblake
I Don't like the Idea of RCD's in use for high Zs readings ,and RCD is there for additional protection , there is way to much reliance on RCD's, if the circuit Zs is within that laid out by the regs then the disconnection time should be met ,without the use of the RCD, what im really getting at is if the RCD fails and the Zs is high then the circuit my well not disconnect but within the max Zs then the required time should be met , see what I mean
P
Paultrim123321
Without telling us the Zs you recorded its hard to give an accurate reply. The RCD could have been selected due to the environment the circuits are in, for example chemistry lab with wet areas and lots of portable equipment with long trailing leads.
I personally would consider installing a type 2 or 'B' type as this is what is recommend for socket circuits unless demand requires otherwise for example heavy inductive loads etc creating high inrush currents.
Using RCDs to 'beat' the system when high Zs are found is all to common, especially without proper further investigation to determine the causes of the original high Zs.
Find out length of run etc, see if you can select 'B' type MCB.
I personally would consider installing a type 2 or 'B' type as this is what is recommend for socket circuits unless demand requires otherwise for example heavy inductive loads etc creating high inrush currents.
Using RCDs to 'beat' the system when high Zs are found is all to common, especially without proper further investigation to determine the causes of the original high Zs.
Find out length of run etc, see if you can select 'B' type MCB.
411.4.9
Whilst some don't like it, its valid to use an RCD if the Zs for the overcurrent device is too high. Although you have to make sure the impedance for a L-N fault will trip the overcurrent protection!
Whilst some don't like it, its valid to use an RCD if the Zs for the overcurrent device is too high. Although you have to make sure the impedance for a L-N fault will trip the overcurrent protection!
4
44ALLAN
And quite a few people I've noticed don't seem to understand that RCD is 'earth fault' and nothing to do with overload or short circuit !
E
Engineer54
Originally Posted by yellowvanman
411.4.9
Whilst some don't like it, its valid to use an RCD if the Zs for the overcurrent device is too high. Although you have to make sure the impedance for a L-N fault will trip the overcurrent protection!
Wow!! These sort of comments really makes one wonder, what's going on out there??
G
GLENNSPARK
Wow!! These sort of comments really makes one wonder, what's going on out there??
yep...
G
GLENNSPARK
yep....
heard it in here a few times....as well as elseware n all...
total lack of understanding...
Points taken, thank you for the replies. The Zs recorded were above the max zs ,1.01 being the greatest reading of one of the circuits. I have flagged it in the report and will try and source the B type breakers. I was just curious as to whether the additional RCD protection would be acceptable in this case. My gut instinct was that it was wrong, but in a TT system you rely on the RCD where the disconnection times cannot be met. Thanks again.
I've brought them today
G
Guest111
some people think RCD's are the answer to everything,Saw it in the paper the other day "RCDto solve syria crisis and end world poverty" oh no wait that's gonna be in next week lol:smiley2:
This Q&A from ESC website
Q4.25
During a periodic inspection, a circuit having RCD protection is encountered and the design information regarding the provision of fault protection is not available. Which value of maximum permitted Zs applies – the value for the overcurrent device (circuit-breaker or fuse) or the value for the RCD (or the RCD element of an RCBO)?
Either value can be used. However, it is essential to confirm that the measured value of Z[SUB]s[/SUB] is not indicative of a defect, such as a loose connection or an inadequate means of earthing.
Regulation number(s)
Q4.25
During a periodic inspection, a circuit having RCD protection is encountered and the design information regarding the provision of fault protection is not available. Which value of maximum permitted Zs applies – the value for the overcurrent device (circuit-breaker or fuse) or the value for the RCD (or the RCD element of an RCBO)?
Either value can be used. However, it is essential to confirm that the measured value of Z[SUB]s[/SUB] is not indicative of a defect, such as a loose connection or an inadequate means of earthing.
Regulation number(s)
- 411.4.4
- 411.4.5
- 411.4.9
E
Engineer54
What does this question prove then??
Depending on the test Zs value it will be clear that there is a potential problem within that circuit. Are you still going to say ...all's fine because i have a cover-all RCD in circuit, that is basically there to afford additional rather than the ONLY or SOLE means of earth fault protection on TN systems!!
Does nobody ever check these day's, as to WHY a circuits ADS Zs value no longer complies?? As RCD protection can never be the sole means of protection on a newly installed TN circuit, the offending circuit must have complied at one time... Or are we now actually installing circuits these day's, that rely on a RCD's Zs protection from the word Go??
D
Dizzy_Maskell
Yes no biscuits though
Depending on the test Zs value it will be clear that there is a potential problem within that circuit.
The answer says
Either value can be used. However, it is essential to confirm that the measured value of Z[SUB]s[/SUB] is not indicative of a defect, such as a loose connection or an inadequate means of earthing.
RCD protection can never be the sole means of protection on a newly installed TN circuit
The question and answer refers to a PIR now called EICR not a new install.
The Reg numbers are there its up to the individual electrician to interpret them as he will.
Totally agree should not be used as a get out for bad design on a new install
R
Rampantchilli
I was told during my studying for 2391, that if max Zs is found to be higher than the max permissible Zs, it is possible to cure in a number of ways, one of which was use of an RCD, the others include additional earthing, change of rating of MCB, change of type of MCB.
Yes, that was at the back of my mind when I tested the circuits. As has been said its not ideal, but away of getting over the problem as long other criteria has been met. I will give the option to change the breakers. Thanks.
E
Edd
Find the Fault as to why Zs is so high, thats what should be done, if not then add earthing or change MCB,,, last resort fit RCD but this really should be a last resort.
Remember this install should have had a respectable Zs when the install was done! if it's now not satisfactory why not get paid to find out why.
If the install had an unsatisfactory Zs at time of install and was left like it then the chances are the rest of the install may have problems..
Remember this install should have had a respectable Zs when the install was done! if it's now not satisfactory why not get paid to find out why.
If the install had an unsatisfactory Zs at time of install and was left like it then the chances are the rest of the install may have problems..
C
C&H
quite often find high zs on C and D rated circuits, not necessarily a fault present on the circuit, just not designed properly. sounds like the OP needs to get himself a B rated breaker and problem solved. 90% of large EICR's i do have high Zs on distribution circuits, it can be hard to meet the max value of a 630a mccb sometimes lol
P
Paultrim123321
People don't realise when a high Zs is measured there is a potential this is due to poor design which will result in a reduced PSC because of increased resistance in conductors for example due to length of run. This will effect the tripping time of the protective device in the event of a short circuit fault and therefore the RCD will make no difference.
4
44ALLAN
I know that Albut not sure some people do!
Read your previous post Paul and do a bit of editing then
D
Deleted member 9648
The point is with this is that it is an EICR....as long as the circuit complies with earth fault disconnection times via either the OCPD or the RCD...and there are no other issues causing the Zs to be higher than the max for the OCPD,no code is applied.
An EICR is simply verification of compliance with 7671...not a take on the original design.
An EICR is simply verification of compliance with 7671...not a take on the original design.
N
nickblake
I have just seen a quote to install RCD's for a small guest house 3 phase QOE consumer unit , this quote was after one of the worst EICR's ive ever seen ,3 DP RCD's to be installed one on each phase , all I want to do is sit and watch what happens when they switch it on should be fun
E
Engineer54
What does this question prove then??
Depending on the test Zs value it will be clear that there is a potential problem within that circuit.The only problem here appears to be in the initial selection of the OCPD,and as it is a 3871 device chances are it is an old install and complied at the time. Are you still going to say ...all's fine because i have a cover-all RCD in circuit, that is basically there to afford additional rather than the ONLY or SOLE means of earth fault protection on TN systems!! It isnt the only means of earth fault protection,the mcb will still trip but not in the required 0.4s,which the RCD will take care of......on many circuits an MCB is the ONLY or SOLE means of earth fault protection,never hear you knocking that.
That will very much depend if or not on any fault that is causing the Zs value to be way out of compliance...
I don't, ...that's because are they are a well tried and proved reliable device, which is unfortunately more than can be said for any RCD device out there!!!
Does nobody ever check these day's, as to WHY a circuits ADS Zs value no longer complies?? As RCD protection can never be the sole means of protection on a newly installed TN circuit, the offending circuit must have complied at one time... Or are we now actually installing circuits these day's, that rely on a RCD's Zs protection from the word Go??It's an EICR on an older installation....likely there is nothing wrong with the circuit, just the initial choice of OCPD,which I say again,likely complied at the time,
Maybe, but then again maybe not, which is why i stated, ...does anybody ever bother to check these day's.
Most non-complying circuit Zs values are easily fixable, but it seems rarely if ever are, when a cover-all RCD is in place!!
G
GLENNSPARK
but on a properly designed circuit...it shouldn`t be......should it.
and thats the nut of it here...
S
Silly Sausage
D
Deleted member 9648
But circuits dont always stay as they were designed,they are altered/extended and change use.Most Zs issues arise only because the wrong OCPD has been selected because that was all that was available,or because it has been changed because of high inrush equipment being installed etc without checking Zs.
That said in the last few weeks I completed a EICR in a 5 year old school where every single sockets circuit was a 4.0/2.5mm radial on 32a/C rcbo's.Nearly all the Zs readings on those circuits came out at 0.9-1.2 ohms,poor design indeed. But while a note was included to that effect no code was applicable because it meets the required disconnection time and therefore complies.
G
GLENNSPARK
had it myself on an ECR at a local football ground..
all c curve....all high.
and the groundsmans hut had socket outlets with no additional protection as well..
Similar threads
OFFICIAL SPONSORS
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
Advert
Thread starter
- Joined
- Location
- Dorset
- If you're a qualified, trainee, or retired electrician - Which country is it that your work will be / is / was aimed at?
- United Kingdom
- What type of forum member are you?
- Practising Electrician (Qualified - Domestic or Commercial etc)
Thread Information
Advert
Thread statistics
Advert
TrueNAS JBOD Storage Server
-
-
-
Understanding TrueNAS JBOD Servers: A Comprehensive Guide- Started by Dan
- Replies: 8