Testing question.

[commandoallan]

Hi All,

I am new to testing having gained my 2391 in November. I have a question that I need help with, I am working on a caravan site on one of the static caravans which is supplied by a TN-S system and have installed a radial 16 amp heating cicuit with six heaters fed from a 2.5 mm2 t+e with the lenth of run being a total of around 20 meters and isolated by six fcu's. I tested the EFLI at the furthest point using the rear of the furthest fcu and got a reading on my megger mft 1552 which is calibrated of 45 ohms.This seems very high and apart from loose connections which I do not belive to be the cause. Could this be a problem with the supply earth as when first testing for ze I got a reading of 80 ohm's. I set my meter on LOOP, isolated the ccu and removed the main earth and tested between earth and live phase, although I am new I belive that these readings are way to high.
Any help or advice would be welcome.
Thank you in advance.
Allan.

 

[jaydee81]

are definately too high for tn-s more like the reading you would expect on a tt system sounds like problem with dno earth

 

[pistolpete]

Surely the hook ups are on their own earth stake or am I getting the wrong end of the stick here, it is a Friday...

 

[spidersparks]

All the statics I've seen are on a tt system but different in the box next to them what was your Ze? r1+r2? these tests should have been done first in any case IMO

 

[chrish_1977]

I would start with the basics first, ze, r1+r2 etc. then you can be sure what you are starting with then make your decisions from there about the readings you get

 

[commandoallan]

Hello,

I think I will measure the ze from the outside hook up point which does not have a earth rod. The earth to the caravan is supplied from the armour off of the two swa. The armour has a banjo with an earth lead off of it to the earth going inside the caravan. If the earth (ze) is no good outside is it worth disconnecting the earth off the armoured and replacing with an earth rod to an MET in the outside hook up point to change system to TT? Do I have to change the mA rating on the rcd located in the outside hook up point if this is done? For some reason both the rcd in the outside hook up point is 40 A 30mA and the rcd in the CCU located in the caravan is the same, so in the event of a fault either one or other or both will trip as there is no proper discrimination between the two rcd's.

Any advice would be welcome.

Many thanks, Allan.

 

[telectrix]

measure the Ze at source. if that is above 0,8Ω, then it's a DNO fault. if the reading is acceptable, then it might be worth your while sinking a good sized rod at the hook-up making it TT, then swap the 30mA RCD at hook-up for a 100mA one.

 

[Engineer54]

What's the betting the system is TT at the source and the Ra is being distributed via the armour of SWA cables!! lol!! There are plenty of lash-ups seen on these caravan sites!! A thorough Investigation is always prudent on a caravan site, be they for static caravans or not!!

 

[telectrix]

What's the betting the system is TT at the source and the Ra is being distributed via the armour of SWA cables!! lol!! There are plenty of lash-ups seen on these caravan sites!! A thorough Investigation is always prudent on a caravan site, be they for static caravans or not!!

sounds logical, E54, what's the betting it's either an old corroded rod or just clamped onto old lead water pipe.

 

[commandoallan]

Hello,

Thanks for all your comments, I will do as I thought and test ZS outside and see what I get, I will also look at source to see if it is earthed there via a earth rod and if not maybe install a new earth rod making this particulal van TT, I will also look at uprating the hook up rcd to 150mA.
Once again thanks for your comments.

Cheers, Allan.

 

[Engineer54]

Hello,

Thanks for all your comments, I will do as I thought and test ZS outside and see what I get, I will also look at source to see if it is earthed there via a earth rod and if not maybe install a new earth rod making this particulal van TT, I will also look at uprating the hook up rcd to 150mA.
Once again thanks for your comments.

Cheers, Allan.

Make that an S type RCD at the hook-up panel, if you have 30mA RCDs in the caravan CU!! A non time delayed RCD will, in all probability not discriminate/co-ordinate well with the 30mA CU RCDs!!

You need to investigate and test first at the source, then at the hook-up panel...

 

[commandoallan]

OK Thaks for that, this will sort out this one caravan although the rest of them will still have the rcd's in the hook up point and CU in the caravan still at 30 mA. Dont think the management will want to upgrade the rest of the hook ups to an S type as if they change this park then they will have to change all the other caravan parks all over the UK as well.

Thanks for your input.

Allan.

 

[telectrix]

OK Thaks for that, this will sort out this one caravan although the rest of them will still have the rcd's in the hook up point and CU in the caravan still at 30 mA. Dont think the management will want to upgrade the rest of the hook ups to an S type as if they change this park then they will have to change all the other caravan parks all over the UK as well.

Thanks for your input.

Allan.

could be a nice little earner. hook up a touring caravan and tour the sites, fixing them all. try not to run foul of jezza clarkson with the caravan though.

 

[charliewilde]

Hi All,

I am new to testing having gained my 2391 in November. I have a question that I need help with, I am working on a caravan site on one of the static caravans which is supplied by a TN-S system and have installed a radial 16 amp heating cicuit with six heaters fed from a 2.5 mm2 t+e with the lenth of run being a total of around 20 meters and isolated by six fcu's. I tested the EFLI at the furthest point using the rear of the furthest fcu and got a reading on my megger mft 1552 which is calibrated of 45 ohms.This seems very high and apart from loose connections which I do not belive to be the cause. Could this be a problem with the supply earth as when first testing for ze I got a reading of 80 ohm's. I set my meter on LOOP, isolated the ccu and removed the main earth and tested between earth and live phase, although I am new I belive that these readings are way to high.
Any help or advice would be welcome.
Thank you in advance.
Allan.

You start with the basic information and I would probably prefer you to make some information so that you will recognize what is the function of that certain matter and how does it affect. However, please make some clarification regarding this matter.

 

[Engineer54]

You start with the basic information and I would probably prefer you to make some information so that you will recognize what is the function of that certain matter and how does it affect. However, please make some clarification regarding this matter.

It would be nice if you could clarify what your actually on about here too?? lol!! Preferably in plain English!!

 

[Des 56]

You start with the basic information and I would probably prefer you to make some information so that you will recognize what is the function of that certain matter and how does it affect. However, please make some clarification regarding this matter.


It seems our American friend is trying to learn about the electrical trade and is doing so through a second language
He seems to be requesting a better description of the supply type etc to help him understand what is being discussed

Or
They are screening Yes minister in LA

 

[commandoallan]

Hello All,

I have delved further into this and have done a ZE test at the hook up point and got a reading of 3.1 ohms which is still above the 0.8 max it should be. I then tested inside the caravan at the ccu and got somewhere around 40 ohms. It stands to reason that I have a dodgy connection between the ccu and hook up point which I will fix by reterminating at the hook up point and retesting at the ccu hopefully with better results, if not then I will run in a new supply.

Thanks all, hope you can understand this as I did not understand the last post by a person from abroad.

Cheers, Allan.

 

[Engineer54]

Hello All,

I have delved further into this and have done a ZE test at the hook up point and got a reading of 3.1 ohms which is still above the 0.8 max it should be. I then tested inside the caravan at the ccu and got somewhere around 40 ohms. It stands to reason that I have a dodgy connection between the ccu and hook up point which I will fix by reterminating at the hook up point and retesting at the ccu hopefully with better results, if not then I will run in a new supply.

Thanks all, hope you can understand this as I did not understand the last post by a person from abroad.

Cheers, Allan.

So what about the Ze value at the source where this hook-up point (and all the rest of the hook-up points on the site) are fed from?? This is where you should have started your investigation, not at the adjacent hook-up point to the caravan your working on!! The 3.1 ohm Zdb value at this hook-up still doesn't comply does with a TN-S system does it?? You will at the very least need to inform the caravan site owner operator/management of your findings. My best guess at the moment (if it is a TN-S system) would be a combination of long cable distances and the SWA earthing connections/continuity at the hook-up positions...

 

[gnselectricals]

Sink a rod & tt the pitch supply box & check for dodgy connections on the. Socket outlet & flex to van.
Don't change any rcd's as that's how it's supposed to be wired
Also check out section 708 & 721 in the regs
Cheers

 

[Engineer54]

Well, well, ...In other words don't bother being an electrician and checking anything, just bung a rod in the ground and TT the pitch!! Makes perfect sense that, 200 ohms to replace 3 ohms!! Oh and who cares, if that there won't be any discrimination between the supply hook-up and caravan RCDs!!

Great advise all round that!! ...Jesus!!