new garage installation

[djjd21]

hi there all I have 2 queries to ask so i'm hoping you'll go easy on me as it is my first post and also my first two jobs which are also for the niceic assesment.

My first is a high Ze result on a TT system which has a reading of 353 ohms for which I nearly had a fit because I had only just upgraded his main earthing conductor to 16mm from what looked like a piece of earth cable from a flex.A little birdie told me that the electrode may not be conducting because of it being too dry in it's hole and to soak the patch in which it resides with water,anyone heard of this or am I just better off buying a new electrode for him and placing it by the side of the old one?

My second is a garage installation which is set 47m away from the main house.Again a TT system is installed with a BS1361 60A distributor cut out,a BS4293 30mA rcd main breaker an 8 way cu with no spare mcb's.What I have done is installed a 40A 30ma rcd in the garage with a 32A and 6A mcb's and supplied the garage cu from the house using three core 6.0mm swa to avoid the volt drop.In the house because there was no spare mcb's and to avoid discrimination between the 2 rcd's i've placed a 63A main switch isolator between the 6.0mm swa and the incoming side of the BS4293 rcd breaker in the house' cu and has also been wired using 6.0mm.All test results are good with only one exception and that being the Zs of the two circuits in the garage which is extremely high at 17.65 ohms compared to that of tn-s and tn-c-s systems which are valued a lot lower.Is this to be expected for this type of system as I have never tested a TT system before and the only test we done on the part p course was on a tn-s system.

Any thoughts or critisiscm is welcomed as i really don't want to screw up this assesment and many thanks to all for their time taken to reply to these queries.

djjd21

 

[Steve3948]

Re Q1, The NICEIC like the reading to be below 200 ohms, however that is not the requirement.

Pouring water on it is not the answer and your inspector would have a fit if you told you had done this, however people do do it

When you did your test was it using the loop tester method, if so did test Live to earth rod conductor, with disconnected from the earth bar.

You could try a new rod but that may be the best reading for the ground conditions in the area you are trying.

Q2.

Hmmmmm,
There a few mistakes here, perhaps one of the other forum members can point them out to you, only I need get a shower before Spooks.

 

[flameport]

353 ohms is too high. Although in theory this is acceptable, in the real world, 100 ohms is a more realistic figure to aim for.
The main earthing conductor on TT systems can be very small indeed - after all, even with a 50 ohm earth stake, it will be carrying less than 5A with a phase to earth short. Nothing wrong with it being oversized though.

Throwing water on it may well reduce the reading - until it dries out again. The correct remedy is either install a longer rod, or multiple rods connected together. Multiple ones need to be spaced apart, 3 or 4 meters would be usual.

The garage does not need 3 core SWA - it should have its own earth rod. 2 core would be fine, as there is no point in exporting a TT earth over that distance.

i've placed a 63A main switch isolator between the 6.0mm swa and the incoming side of the BS4293 rcd breaker

Do you mean that this length of SWA has no protective device, other than the main incomer? If the answer is yes, then you won't be passing that assessment.

Zs of the two circuits in the garage which is extremely high at 17.65 ohms

For a TT system this is fine - that is why the RCD is required. What are the Zs readings in the house? Presumably there are metallic service pipes which reduce the reading (as you had 353 ohms at the stake?)

 

[ricicle]

How long was your Part P course.Have you completed the City and Guilds 2391?

 

[DESL]

Q1 try here http://www2.theiet.org/Publish/Wire..._multiple_earth_electrodes_in_tt_dwelling.pdf

Q2

What were the values of (R1+R2) for garage supply and Ze on main board.

What protection is there for the garage supply from main board you say 63A Islotor it should be a switch fuse.

Edit: beaten to it again

 

[Spark123]

i've placed a 63A main switch isolator between the 6.0mm swa and the incoming side of the BS4293 rcd breaker

Do you mean that this length of SWA has no protective device, other than the main incomer? If the answer is yes, then you won't be passing that assessment.

This doesn't sound good tbh. Where is the overcurrent protection? Where is the fault current protection also? Protection against indirect contact is normally by RCD in a TT system.

 

[securespark]

The MEC can be as low as 2.5mm².

As for the RCD's, two 30mA RCD's will not discriminate.

 

[djjd21]

thanks to all of you that have replied to the original post.just a couple of things that may be a bit confusing to you.first of all the 2 queries are seperate installations.the garage installation's Ze value at the house is 76.3 ohms and not 353 like the other.

in answer to flameports reply i used 3 core swa to take both phase conductors and an earth to the garages own cu unit which consists of a 40A,30ma rcd 1no 32Amcb for a 6 outlet ring main and a 6Amcb for the lighting does this not class as overcurrent protection.the isolator has been put in place just to cut the power to the garage consumer unit.

in answer to ricicle question my course was 7 days in total and i'm planning on the 17th edition exam early in the new year.

in answer to desl question the (r1 + r2) test in the garage was 0.17ohms for the ring circuit and 0.56 ohms for the lighting circuit and the Ze at the electrode at the front of the house is 76.3ohms

 

[nozspark]

Firstly - if you use this installation for your assesment then you should fail..

Secondly...

All circuits in a TT installation should be RCD protected (if in the same CU then the usual practice is to use a 100A 100mA time delay RCD as the main isolator and then a 30mA RCD for the other circuits.

In my opinion you should also have only used 2 core SWA to the garage, terminated it into an insulated enclosure right next to the CU, continued the Live and Neutral out through the box and into the isolator (30mA RCD) do not link the earth (you are not allowed to have 2 earths which could be at different potentials within the same enclosure)!!!! You should then have provided a local rod for the garage.

If you have a Ra of 300 odd ohms then this is high for an assesment, as said by others you should add another rod, but place it a good 4 to 6 foot away from the existing one. If the main earth is protected against corrosion, but not mechanically protected you could use 4mm earth.

 

[ricicle]

my course was 7 days in total

 

[DESL]

the isolator has been put in place just to cut the power to the garage consumer unit

But this supply to garage needs a CPD

How have you connected supply to incoming side of main RCD

If I was doing the job I would split the tails via a Henly block one set of tails to main CU and the other to a switch fuse or a metal enclosure containing a suitably rated MCB to supply garage in which it would be suitable for terminating the SWA.

As you described the existing set up there is NO protection for supply to garage escept main cut out

 

[nozspark]

DESL

amd it would still be wrong, being a TT installation all circuits require RCD protection and having a metal enclorure without RCD protection up stream on a TT is a No-no

 

[ricicle]

being a TT installation all circuits require RCD protection

Not strictly true if you read the regs - the RCD is the prefered method to ensure disconnection.....

 

[nozspark]

OK then I admit that, but would you??

 

[DESL]

Point I was trying to get over was the fact that OP had no protection for garage supply.

I take the point of having RCD protection for this supply to achieve disconnection times with a high Zdb

So do away with RCD in Garage CU and feed the garage using RCCBO at main supply