Buried SWA cable fault finding

danielz000 · Sep 10, 2021

Hi all,

My first post here, thought I'd see if it was possible to get some advice or tips...

Basically, I had a large amount of block paving and large concrete base laid at the far end (25m) of my garden a few months ago. Under this I had 3 core 16mm SWA laid.

(I most definitely can't vouch for the quality of the work done by any of these guys)

More recently me and my brother built a large garage on the concrete base and had an electrician add a new CU in the garage and connect the other end of the SWA to an MCB in the CU in the house.

Today, after about 2 weeks of not having any problems, the RCD in the house tripped out and wouldn't switch back on unless I turned off the MCB to the garage. It's worth noting, it rained fairly heavily last night.

After trying a few different things, I've eventually disconnected the SWA from the CUs at both ends.

I've tested for continuity across all cable/ armour combinations and noticed there's continuity (beeping) when I touch the live and armour.

Is my assumption that someone has damaged the cable, breaking the live wire insulation and water has entered this and created a circuit between the live and the armour a reasonable one? Are there any other ways this continuity could occur given both ends are now fully disconnected?

Assuming this assumption is correct, I'm curious if there's a simple way for me to locate this fault without digging up 30m of block paving?

For example, I was wondering if, by using a multimeter and testing the resistance across the live and armour at both ends I might be able to work out how far down the wire the break is?

So maybe if I got a resistance 10 times higher at one end than the other I can assume the damage to the 25m long cable is 10% (2.5m) from the end of the cable with lower reading?

I guess I don't need to be super accurate and that there are industrial tools that could accurately locate the fault, all I really want to know is roughly were to start digging in the hope of keeping the damage to the paving to a minimum.

Thanks in advance for any help or suggestions.

Dan

marconi · Oct 29, 2021

Update: Freddo has been very busy but managed to attend the sports ground with the defective swa cable last weekend. We had some conversations on my test rig and how to use it in the private area which I have copied below:

Good morning. A 25mm2 conductor has a resistance of 0.000727 Ohms/m. When a current of circa 75A flows through it the voltage drop is 75 x 0.000727 = 0.055V/m. Not much. A 40m length of 25mm2 conductor will drop a voltage of 40 x 0.055 = 2.2V Thus when you measure the near and far voltages with your dvm you will need to drop down in range scale in order to obtain the necessary resolution. My dvm has a 2000mV range which would be ideal for example.

We are using a variation of the 4 wire resistance measurement method. It is important that the 'voltage sense' connections are on the 'outside' of the near and far current loop connections. See para 2 of :

4 Wire Testing | Resistance Measurement to within 1mΩ | How-To - https://www.camiresearch.com/Campaigns/Web-Articles/4-wire-testing.html

So at the far end the Henley is wired with the middle terminal to black and right to red - the current loop - and the sense connection blue is made to the left of the black to the middle terminal.

A nice feature of this set up is that the near readings will be negative voltages and the far ones positive due to the negative terminal of the dvm being connected to the point of fault. You should see this when you do the on arrival survival tests with the three 3.3R test resistors.

After measuring R-Y, I recommend you swap the SWA for the Yellow and take the R-SWA measurement. Finally swap the Yellow for the Red and take the Y-SWA measurement.

You need to know the length of the cable in order to use the data to divide it into near and far. Say Near is 250mV Far is 1850mV Distance to fault from start of cable is length of cable x 250/(250 + 1850). My example in the image indicates the cable is damaged only in one place. The most likely region is as I have shown.

When I have the voltage measurements I will do the sums.