failed Zs

[nicsin02]

Hi guys, I am currently testing a large hotel (over 300 rooms) it is only the 4th level we have to test this year (65 rooms) each room has its only 5-way d/b consisting of a 32 & 6 Rcbo type B, each of these d/b's are fed from different larger d/bs installed in various fire escapes along the corridor, I have found that these small d/b's in the rooms Zdb's are getting quite high the further away they are getting from their supply d/b some of them even going as high as 1ohms therefore making the Zs of the 32a circuit far too high, ie 1.76. Ohms etc.......

1. Is a mac Zs the same on a 60898 as a 61009?

2. If the max Zs test fails will they use the fact it is protected by an RCD so Zs is mute or is this Rcbo merely used as additional protection not fault protection?

Thanks guys

 

[Sintra]

The max permissible Zs on a 61009 is 1667 ohms although on a TN system they will be used as additional protection therefore IMO the maximum permissible Zs of the equivilant 60898 should be met.

 

[telectrix]

one solution might be to reduce the rating of the MCB to 25A or 20A. in a hotel, i would imagime that the current demand in each room is pretty small.

 

[Engineer54]

In circumstances such as you find yourself in, the Zs of a circuit MCB protective device is the value you follow and not the Zs value an RCD brings. This is a hotel, which brings additional hazards such as fire risks etc!! Also the installations within each of these rooms, will be used by paying customers, who's safety would be paramount to the hotels owners...

 

[nicsin02]

Thanks guys, knew I was right, just wanted it confirmed, so Is it only a TT system that we would use an RCD as fault protection because few months back I was testing food distribution place and 50% of the outside lorry sockets failed Zs test and the customer was adamant that other companies had passed as these circuits were RCD protected?

 

[telectrix]

Dith a TT system, unless you can get sub 1 ohm Ze readings, you'll never meet the Zs figures formost OCPDs and so you have to rely on ther |RCd

 

[Guest112]

Thanks guys, knew I was right, just wanted it confirmed, so Is it only a TT system that we would use an RCD as fault protection because few months back I was testing food distribution place and 50% of the outside lorry sockets failed Zs test and the customer was adamant that other companies had passed as these circuits were RCD protected?

yes the results would pass as rcd protected although I would confirm that the ze from the db subblying the outside sockets was high resulting in a high zs to confirm the circuit isn't producing the high reading

 

[Guest55]

its an inspection , yes ?
then do nothing other than record your measured values in the report , along with any defect code you think is appropriate.
dont start worrying about any necessary changes to get it fixed , sort that when the inspection is completed.
you may find high Zs readings are the least of your worries when testing a hotel. ;-)

 

[somersetsparks]

In this situation with rcd protection on the affected (poorly designed) circuits
It would be the oher factors that would concern me,
I.e volt drop
And obviously these circuit are well over the maximum length values given in BS7671, therefore if there were a short circuit on a cable, could the MCB be relied upon to break the circuit in time?

 

[Risteard]

Whether we like it or not, it is permissible to rely on the RCD protection with a high Zs (or supplementary bonding is another one). However, it would be worth confirming the line/neutral loop impedance as well as the earth fault loop impedance.

 

[i=p/u]

Badly designed, corners cut. C2... Potential hazard if rcd sticks to itself ???

 

[Deleted member 9648]

If it's a TN system at the design stage it would be unacceptable to rely on an RCD for earth fault protection,the circuit must be designed and installed with the max Zs of the OCPD in mind.
If you are carrying out an EICR on an existing install then the only consideration with Zs as far as coding is concerned is compliance with disconnection times,and if there is a working RCD in circuit then disconnection times are met,so no code is applicable.
Other issues which may result from the reason for a high Zs may warrent a code.....but not because of Zs and disconnection times.

 

[Guest55]

i'd give it a C3 for excessive Zs , no danger but requires improvement.

 

[Deleted member 9648]

i'd give it a C3 for excessive Zs , no danger but requires improvement.

In order to code it would have to not comply....if the disconnection time are met it complies.

 

[Guest55]

dont care , my decision is final lol.
;-)

 

[Silly Sausage]

i'd give it a C3 for excessive Zs , no danger but requires improvement.

Should start with capital I and a comma after danger.

Re OP: I've seen similar in a hotel.
In my case, the geezer I was testing with was giving old code 2s for RFCs on 2.5, on rooms with their own DB, protected by 20A breakers! Never been any reports of nuisance tripping.

 

[Guest55]

Should start with capital I and a comma after danger.


the geezer co-worker ?

breakers mcb ?

sloppy.
;-)

 

[Silly Sausage]

sloppy.
;-)

Grammatically, syntactically and spelling correct, my friend.
Slang, in these modern times, is perfectly acceptable.


Sod this I can't keep it up!

 

[pete555]

In order to code it would have to not comply....if the disconnection time are met it complies.

problem with RCBO is not only does it have to comply with Earth fault but overcurrent as well that will bring your loop impeadence down to meet trip time on over current

 

[Engineer54]

The problems come, when the RCD covering these non compling Zs circuits fails!! And as this is a multi-storey hotel, i know what i'd be doing!! lol!!

 

[Deleted member 9648]

problem with RCBO is not only does it have to comply with Earth fault but overcurrent as well that will bring your loop impeadence down to meet trip time on over current

Zs has nothing whatsoever to do with overcurrent,only earth fault current.

 

[Guest55]

And as this is a multi-storey hotel, i know what i'd be doing!! lol!!

ordering room service and emptying mini-bar ?

 

[Engineer54]

Room service maybe, but the mini bar would still be pretty much in tack, i'm not a big drinker!! lol!!

I was thinking more of, well and truly covering my arse on that certificate!! lo!!

 

[i=p/u]

What happens when no cpc on older lighting circuits , how do you test if this circuit will disconnect ? Do you just add up R1+Rn to ze

 

[Guest55]

What happens when no cpc on older lighting circuits , how do you test if this circuit will disconnect ? Do you just add up R1+Rn to ze

well there is no Zs without an earth.
and a dead short between L & N will disconnect device fairly quickly without the need for any testing

 

[telectrix]

in this case the rcd would disconnect (assuming it works). you would not get a fault to earth except through a body.

 

[nicsin02]

I am just going to put these Zs fails a C2 fault and recommend the mcb get downgraded to either 16/20amp, I've always been led to believe that a 30mA RCD is for additional protection on a TN arrangement, I do alot of new build house tests where the full d/b is RCD protected and it would make the Zs test pointless if I went down the line of relying on the RCD for tripping times?

 

[Deleted member 9648]

I am just going to put these Zs fails a C2 fault and recommend the mcb get downgraded to either 16/20amp, I've always been led to believe that a 30mA RCD is for additional protection on a TN arrangement, I do alot of new build house tests where the full d/b is RCD protected and it would make the Zs test pointless if I went down the line of relying on the RCD for tripping times?

I understand what you are saying....but a new build TN system should be designed with OCPD disconnection times in mind. An EICR is only concerned with compliance with Bs 7671. IMO you would be wrong to code 2 a circuit which complies with the required disconnection times,reg 411.4.4 applies,and clearly states an RCD is a permitted device for earth fault protection.
An EICR is not about personal preferences.....good luck with claiming a circuit which complies with 411.4.4 is 'potentially dangerous'.

Edit
Just to add -you could always include a recommendation in your report that the issue is addressed,giving reasons. But a code 2 is an automatic 'unsatisfactory',potentially forcing the client to part with a wad of cash. It's going to be hard to justify your report when the client finds out he's spent a lot of hard earned on an install which actually complied with the regs.

 

[ojchurch]

I agree with you on new builds, should be designed to trip in time regardless of RCD. You would still need to carry out the test though to confirm CPC is continuous.
I was replying to nicsin02 (not great at this forum thing!):
I am just going to put these Zs fails a C2 fault and recommend the mcb get downgraded to either 16/20amp, I've always been led to believe that a 30mA RCD is for additional protection on a TN arrangement, I do alot of new build house tests where the full d/b is RCD protected and it would make the Zs test pointless if I went down the line of relying on the RCD for tripping times? !):

 

[telectrix]

as wiepuller said, a code C2 is a non satisfactory, potential danger, so a bit OTT. i'd go with C3, improvement recommended, and as your recommend downrate MCBs from 32A to 20A.

 

[Deleted member 9648]

I agree with you on new builds, should be designed to trip in time regardless of RCD. You would still need to carry out the test though to confirm CPC is continuous.
I was replying to nicsin02 (not great at this forum thing!):
I am just going to put these Zs fails a C2 fault and recommend the mcb get downgraded to either 16/20amp, I've always been led to believe that a 30mA RCD is for additional protection on a TN arrangement, I do alot of new build house tests where the full d/b is RCD protected and it would make the Zs test pointless if I went down the line of relying on the RCD for tripping times? !):

It can be either additional or earth fault protection,have you looked at 411.4.4?....which deals with TN systems.
And a new installation and initial verification is a different animal to an EICR....where the only consideration is compliance with 7671.

 

[ojchurch]

It can be either additional or earth fault protection,have you looked at 411.4.4?....which deals with TN systems.
And a new installation and initial verification is a different animal to an EICR....where the only consideration is compliance with 7671.

Maybe I haven't been clear, sorry, I don't often post on forums so am probably not used to the etiquette etc. I was responding to nicsin02's post which I then edited in after. Must get used to these symbols on the mobile format, reply with quote isn't obvious there.

I am fully aware what regulation 411.4.4 says and if I was carrying out an EICR to BS 7671 I would pass the circuit in question as long as all other requirements were met. Also obviously I would have to be sure the high reading was legitimately down to the combined cable runs & not other reasons. I would however detail my findings in the report and try and explain to the client in laymens terms what I had found and the ramifications.

I realise now that I didn't read the post correctly, I was on a tea break! I was trying to say that you still need to confirm continuity of CPC but nicsin02 was talking about not having to take the EFL reading, not confirming continuity of CPC. My bad.