Tight fit or room to spare?

[LeighSawczyn]

Does 6mm2 SWA fit into 25mm Blue MDPE Pipe. The idea being to protect the cable from damp soil or water ingress when it is buried, not for mechanical protection. No tight radii to worry about just a 10-15m straight run.

 

[Knobhead]

Thanks Dave. I've maintained buildings much older than that and never seen unserved cables before so it's interesting to hear about them.

Daft question, but would I be right to assume that they are only for dry areas?

The cables in Lucians photographs are acording to the information are PILC.

Most old PILC cables are first wrapped tarred paper and then a layer of tarred jute. Over this the first layer of armourings followed by another layer of tarred jute. A second layer of armourings over wrapped with tarred hessian. Just to add to the fun it would then be whitewashed to stop it sticking together on the drum.

Stripping it you set off covered in whitewash, followed by tar. The only way to clean the tar off the armours and lead sheath is with paraffin and a blowlamp. By the end of the day you were a fire hazard and reeked to high heaven.
We nearly burnt the EMEB training centre down with a bit of a cock up with the paraffin rags and blowlamp.

 

[davesparks]

If you could Dave. You know my interest in the history of our trade.

I will do my best to remember, I think we've got a show going in there in March. The installation is a bit of a shambles at the moment though. As usual the remaining bits of the original installation are well installed etc, but the more modern abortions are a disgrace.

 

[ImpededLoop]

The cables in Lucians photographs are acording to the information are PILC.

Most old PILC cables are first wrapped tarred paper and then a layer of tarred jute. Over this the first layer of armourings followed by another layer of tarred jute. A second layer of armourings over wrapped with tarred hessian. Just to add to the fun it would then be whitewashed to stop it sticking together on the drum.

Stripping it you set off covered in whitewash, followed by tar. The only way to clean the tar off the armours and lead sheath is with paraffin and a blowlamp. By the end of the day you were a fire hazard and reeked to high heaven.
We nearly burnt the EMEB training centre down with a bit of a cock up with the paraffin rags and blowlamp.

Tony,

Thanks for your reply and explanation, it's interesting and much appreciated. My previous workplace had plenty of PILC with the tarred Hessian wrapping, but I've never seen them without it.

Very interesting to hear about the use of whitewash to stop the cable sticking to itself on the drum. I have to admire those people who installed the stuff.

Thanks again

 

[Engineer54]

Any SWA cable that's been installed into a buckshee trench that hasn't had the sharp stones and other debris removed from the base of the trench and then just backfilled with the excavated earth is almost certain to have it's outer sheath compromised!! There are good reasons behind how a cable trench should be constructed with the correct procedures and materials....

Basically the above is unfortunately a description of what normally applies to domestic buried cables, that are not run in a suitable duct. You would be surprised, even when direct buried SWA cables have been run in professionally, how often a sheath test fails!! And yes it does then need to come out, repaired and reinstalled!!

 

[Knobhead]

And you’re not the most popular person in the world when you tell the cable lads its got to come out.

We would get cable contractors if there were many runs. For the odd runs our quarry production lads were great, they’d done the job many times and really did know what they were doing. Believe me, get them together en-mass and it’s a frightening sight.

They were quite happy with a pull as on Wednesday afternoon as the quarry and crusher shut down for maintenance so the ideal time. A pull was overtime for them plus it was “job and knock”. They got the full afternoon’s overtime and knocked off once the job was done. I dare anyone to tell them the cables got to come out again, your life wouldn’t be worth living.

 

[Tidy Max]

Was questioning to lead the posts the correct way, I don't deal or have experience with HV so would be out of the conversation but if he's talking about running a 6mm 3core to his shed under his back garden then its all usually manual backfill and wouldn't warrant a sheath test as most domestic sparks don't have the appropriate tester to do so, no column on their test sheets for it either... wasn't questioning the testing of the sheath but the circumstances of the install if he's doing a sheath test as this tends to be past your domestic set ups thus curiosity got me, not often a test mentioned on here.

I bet you can drop this conversation into the forum and 90% of the guys who have buried cable on here will not have done a sheath test, I've not done it often myself but thats more to do with the fact I don't bury many cables. I questioned him because of his strange question at the beginning of using a blue pipe as a sheath - its one of them things that ring alarm bells, so had to clarify exactly what kind of install he's doing, he seems to do a test that most on this forum will never have even done yet asks a weird question to boot when standard ducting would be the way to go, even if he had a 10mm clearance he would not get that swa though that pipe as both would have natural bending in them from storage and the friction would be high over 15m length.

@OP - I wouldn't be going anything less than a 50mm duct to drag a 6mm swa through, it will also allow for future issues to be easily rectified if needed to change the cable.

What is the special test equipment? Would you not just use an insulation resistance meter?

 

[Darkwood]

What is the special test equipment? Would you not just use an insulation resistance meter?

Its the reason I was asking as to what the voltage was and what kind of work he was actually doing, not all sheath teats can be done with a bog standard Insulation test as this is just a 'pulse method', some companies have requirements that mean a 'Time-Resistance method' is the test needed and most of our combi-testers don't have this option and are not designed to say maintain an Insulation test of 500v/1000v DC for 1minute.

The OP gave no indication as to the nature of the work or who he was contracted under and this may well have sent the thread in a totally different direction. I use Time-Resistance method occasionally myself for motor testing during maintenance checks as it has many advantages against the spot reading method and overcomes a few issues that effects your readings.

 

[Engineer54]

And you’re not the most popular person in the world when you tell the cable lads its got to come out.

We would get cable contractors if there were many runs. For the odd runs our quarry production lads were great, they’d done the job many times and really did know what they were doing. Believe me, get them together en-mass and it’s a frightening sight.

They were quite happy with a pull as on Wednesday afternoon as the quarry and crusher shut down for maintenance so the ideal time. A pull was overtime for them plus it was “job and knock”. They got the full afternoon’s overtime and knocked off once the job was done. I dare anyone to tell them the cables got to come out again, your life wouldn’t be worth living.

I've become quite used to being the bearer of bad news, ...at the end of the day it's their work and therefore their problem, not mine!!

We undertake sheath tests after the wet sand fill and allowed to compact around the cable, so not quite as bad as testing after complete backfilling has taken place... lol!!

 

[Knobhead]

What is the special test equipment? Would you not just use an insulation resistance meter?

I used a bog standard Megger @500V.
You’re not testing insulation as such. You’re ensuring the integrity of the outer serving. We used quarry sand straight out of the washing plant, sopping wet and it soon consolidates around the cable. The damp will soon find its way in to any damage.

The insulation tests Damien mentioned would usually be of five minutes duration, sometimes more for long runs as they take time to charge. Our Megger could read to 100TΩ, 100,000,000,000,000Ω.
It gave one or two some nasty bites. The best one was getting the engineering manager, his own fault for crossing the test area barriers. I’m sat in the substation watching the reading gradually rising when it suddenly shot down and I heard a squeal. Served the silly bugger right.
Others got caught trying to take the leads off before the Megger had discharged the cable.

 

[Engineer54]

I was a little surprised the first time i ever mentioned the ''Sheath Test'' on the forum, very few had ever heard of the term let alone know how to perform or indeed ever conducted a sheath test. We've since had people here trying to conduct a sheath test with the SWA connected to switch boards etc, and those asking what IR value is acceptable for a SWA sheath test!!