Zs on ring

[SparkJ]

I have added a spur to a ring main in a small 2 bed modernish (20 years old?) maisonette.

Earthing is TNC-S, water and gas bonding is in place.

Consumer unit is a Hager unit with 30ma RCD covering all circuits. The ring circuit has a B32 MCB to protect against overload.

My problem is Zs readings. According to the OSG table 2D max Zs value is 1.16 ohms for a 32A 60898 breaker.

The Zs readings for all of the sockets on the ring apart from the spur that I have installed range between 1.06 and 1.1 but the spur comes in at 1.19. I have checked all of the connections behind all of the sockets and they are tight. There are no FCUs on the ring or any other connections that I can find.

The RCD trips at 1 times rated current at 37.6ms at 0

 

[dave b]

hi there,

the max loop impedance for that m.c.b as you said is 1.16ohms in table 2d. because you are just oustside this measurement, you are required to fit rcd protection for automatic distconnection and not rely on automatic distconnection on the mcb alone.

so the answer is yes. the rcd fitted will give the distconnection required in the event of a fault. i would still record the measurent and insert on the certificate.

hope this helps

dave

 

[The Godfather]

You could back track from the spur and check some/all of the sockets for a loose connection (and or check the board, MET).

This could well bring you in with BS7671.

However the error is 0.026%, probably exceeding most meters accuracy.

The Godfather

 

[batty]

I bet if you checked with another meter it would be a lot less. I have four loop testers some are mft and one is just a loop tester. Doing some work at the weekend at my house I had readings between .38 and 1.16 ohms on one socket so the accuracy of these meters does make me wonder.

Batty

 

[Markiemarque]

Hi,

I am quire new to all this so I may be a little off in my suggestions, but hey .... it's a learning process.

What were the readings for r1, r2 and rn? You can use these to calculate the length of the cable runs to make sure you are not getting bonkers readings.

What is the calculated Zs ... Ze + (R1+R2)? You get R1 + R2 from the third ring continuity test and you do Ze at the origin ... is the calculated Zs less than your reading at the socket?

Hope this helps

Mark

 

[The Godfather]

One should consider before adding a spur to any ring main, is to check the Zs at the spur socket.

Don & His Boys

 

[SparkJ]

Thanks for your input Markiemarque and Batty.

I have calculated the lenght of the runs using table 9a in the OSG and get around 20m which is feasible.

Calculating Zs gives me a value of just on 1 ohm. As you say the meter is probably not very accurate measing Zs.

I guess as there is a functioning RCD on the circuit the problem is academic. But it is good practice to get an understanding of how it all hangs together.

 

[green-hornet]

You have calculated your Zs readings on a circuit that includes a 13 amp 1361 fuse(in the fused spur)

I would not include the spur in your final ZS readings as this would not be accurate.

The fused spur is in effect a seperate distributed circuit and as such all readings should be taken back to the supply of the circuit intsalled.

The readings you have given whilst on the high side are not outside the maximum values so I would say that the installation is fine.

 

[batty]

Thanks for your input Markiemarque and Batty.I have calculated the lenght of the runs using table 9a in the OSG and get around 20m which is feasible.

Calculating Zs gives me a value of just on 1 ohm. As you say the meter is probably not very accurate measing Zs.

I guess as there is a functioning RCD on the circuit the problem is academic. But it is good practice to get an understanding of how it all hangs together.

If your meter will do a loop p-n see what that reading is because if that is high then you have problems. Generally it will be a lot lower as meter puts a good current down circuit and resistance of cables will also be lower. What is the ZE of property.

Batty

 

[SparkJ]

0.2 ohms according to my meter

 

[a1spark]

I note you say the property is approx 20 years old...if it is in fact a bit older than this you may find the csa of the t&e is 1.0mm, accounting for the higher than expected calculated Zs........also take into account you are using the No trip settings on your loop tester, these are notorious for giving higher results than actual calculated results

a1spark

 

[spinlondon]

Where are you taking the measurments from. Sometimes you can get a much lower reading if you place the meter probes on the terminals at the back of the socket. Rather than using the pin holes at the front of the socket.