Insulation test L to E long length of SWA

[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

 

[brianmoooore]

Are there any large loads in the lodge?

 

[sidekick]

Nothing unusal other than a hot tub.

 

[brianmoooore]

What's the heating, water heating and cooking?
EV charge point?

 

[sidekick]

What's the heating, water heating and cooking?
EV charge point?

No Ev charger. Will confirm all load calcs when I call back tommorow.
Are you thinking possible over load of 63A breaker and/or 16mm 2 core swa

 

[brianmoooore]

More than possible, especially since you'll be half way there if the hot tub's a 'proper' one.

 

[sidekick]

More than possible, especially since you'll be half way there if the hot tub's a 'proper' one.

Thank you for your help. Will double check everything tommorow and try to find out what was on at the time of tripping. Still need to retest the cable though as that stands out at the moment.