Insulation test L to E long length of SWA

[James]

That'll be Meg ohm then
I had a college teacher who said mega ohms I cringed every time he said it.

Hate to say it Westward, but mega Ohm is the correct way of defining it.

 

[ferg]

View attachment 113647
Hate to say it Westward, but mega Ohm is the correct way of defining it.

To be fair to Westy it wasn't adopted until 1873 which is probably after he went to college.

 

[sidekick]

I should also say that this swa cable has a 63a mcb at the front end and is randomly tripping ?

 

[brianmoooore]

Is that with whatever's at the far end switched off?

 

[sidekick]

Is that with whatever's at the far end switched off?

No tripping when sub main switched off as that was only briefly while testing .
Live and earth disconnected at front end (neutral still connected) and mains switch and all breakers off at back end to test. Random tripping was when all on (2 to 3 times in last 24 hrs)

 

[pc1966]

I should also say that this swa cable has a 63a mcb at the front end and is randomly tripping ?

If it randomly trips with the load-end of the SWA isolated then it confirm the IR results that the cable is damaged somewhere.

Most likely you see your 0.7 M ohm result after it has gone "Bang!" internally and you go to test it, then after time water creeps back in and it goes bang once more, etc.

There are techniques to help locate where along the cable a fault it, but some depend on it being a low and stable short (so you can see the relative resistance to a fault from the cable ends to guess approximately the ratio of cable to it), and others require expensive time-domain reflection sort of equipment (to look at the waves bouncing around just after it clears a fault). Has there been any civil works, digging of gardens, etc, recently? That might point to a likely location.

But it could just be poorly installed cable without sand/fine gravel around the cable resulting in a stone crushing it over time.

 

[R-fur]

Underground and laid about 20 plus years ago.

You say it is about 100m long, walk the route if you know it and look for possible disturbances, like cultivated ground, new fence posts etc. A common method of localising is to dig down half way and cut and test the cable. Then repeat until the fault is located. But be careful to make sure that you cut the correct cable. You have to risk assess, are there other cables in the area? Can they be isolated while you cut? It just a matter of patience and thinking. I know some people on here don't like the idea of joints, but done correctly they should be as good as the cable. I come from a DNO background and have used this method many times.

 

[pc1966]

I know some people on here don't like the idea of joints, but done correctly they should be as good as the cable.

100% agree that if you can find the damage it is not too difficult to fit proper resin joints and they are good for decades underground.

If you can find the location!

 

[davesparks]

That'll be Meg ohm then
I had a college teacher who said mega ohms I cringed every time he said it.

The standard prefix for million in the SI system in accordance with international standards is mega.

 

[timhoward]

There are techniques to help locate where along the cable a fault it, but some depend on it being a low and stable short (so you can see the relative resistance to a fault from the cable ends to guess approximately the ratio of cable to it), and others require expensive time-domain reflection sort of equipment (to look at the waves bouncing around just after it clears a fault). Has there been any civil works, digging of gardens, etc, recently? That might point to a likely location.

^^ This is the bottom line.
Have a good look for any recent activity.
If that fails, you might find an older sparks locally that still has a wind up megger that does Varley loop tests, or someone that has a Megger Bridge, or an even older person who has a Post Office box! This all requires
1) you know the route
2) there is one undamaged core
3) the fault to earth has a stable resistance (as @pc1966 said)

If that fails you are into hiring a TDR- again you need to know the route for this to help, and therefore extra cable tracing tools to determine the path might be needed.

On jobs like this that I've done it tends to end up either
-easily finding the reason (shiny new fence post) or
-identifying roughly which section of the cable is faulty without finding the reason, making cuts, establishing the remainder is fine, and running a new section.

 

[sidekick]

If it randomly trips with the load-end of the SWA isolated then it confirm the IR results that the cable is damaged somewhere.

Most likely you see your 0.7 M ohm result after it has gone "Bang!" internally and you go to test it, then after time water creeps back in and it goes bang once more, etc.

There are techniques to help locate where along the cable a fault it, but some depend on it being a low and stable short (so you can see the relative resistance to a fault from the cable ends to guess approximately the ratio of cable to it), and others require expensive time-domain reflection sort of equipment (to look at the waves bouncing around just after it clears a fault). Has there been any civil works, digging of gardens, etc, recently? That might point to a likely location.

But it could just be poorly installed cable without sand/fine gravel around the cable resulting in a stone crushing it over time.

Thanks for the info.
The actual swa supplies a holiday lodge which is currently let and power trips for the tenant randomly. No reccolection if weather condition at the time. Everything stays on at the lodge submain but the main 63a breaker at supply end trips.
Tested with load switched off produced the 0.7 m ohm but load never been off long enough to see if trips with load off.

Tenant moves out tomorrow and I will go test again, so any additional advice/tips/tests welcome as dont want to put landlord down route of digging etc if any way I can confirm fault and make an easier task of it.

 

[brianmoooore]

If the tenant is moving out, there shouldn't be any large loads at the lodge end (electric heating off?), so if the MCB trips then, it's likely to be the cable.

 

[sidekick]

If the tenant is moving out, there shouldn't be any large loads at the lodge end (electric heating off?), so if the MCB trips then, it's likely to be the cable.

Good point but it is so random it may not trip untill next tenant in anyway. Tommorow is my window to test and confirm cable or not.
Although I have advised lodge should stay vacant until issue resolved.

 

[Rockingit]

You can potentially narrow this down with some calculation - by using a normal resistance test you should in theory get a different reading from each end of the cable (well, unless the damage is half way..) at which point you can look up the Ω/m for that csa of cable and if you take the cpc as a constant datum then you can get a gauge for where to start looking to dig. A couple of hours worth of head scratching with a calculator and a torpedo repair sounds a lot less painful than a total replacement. That said, the whole thing might be f'd if water has crept through all of it, but it's worth the go.

 

[sidekick]

You can potentially narrow this down with some calculation - by using a normal resistance test you should in theory get a different reading from each end of the cable (well, unless the damage is half way..) at which point you can look up the Ω/m for that csa of cable and if you take the cpc as a constant datum then you can get a gauge for where to start looking to dig. A couple of hours worth of head scratching with a calculator and a torpedo repair sounds a lot less painful than a total replacement. That said, the whole thing might be f'd if water has crept through all of it, but it's worth the go.

Thanks for that