Sap 7 or Gap 7 as the bedding materials for Underground unenclosed cables

[Tiger]

Hi Everyone,

Just wondering is it acceptable to have Sap 7 or Gap 7 as the bedding materials for Underground unenclosed cables ?

3.11.4.2 Unenclosed cables
Any Category A or Category B wiring system that comprises cables not
installed in a wiring enclosure shall be laid on a bed of not less than 50 mm
of sand or friable soil, free of sharp stone, and covered by not less than
50 mm of the same material.

Cheers,
Tiger

[JamieP]

Is it sand or friable soil? Is it free from sharp stone?

If yes, then it's compliant

If not, then it's not

I'm not super familiar with the products but just had a look at what I could online and they do look like they probably meet the definition of sand or friable soil

[AlecK]

The "AP 7" bit means "all passing 7 mm
So the material has passed through a grille / sieve with holes 7 mm square.
for all "AP grades; some of the particles will have a dimension > the 7 mm nominal size; having slipped through endways.

AP 7 is probably fine for nice round pebbles.
But the "S" means scoria, which has sharp corners - and that won't comply with "free of sharp stone".

When it comes to bedding cables, the point is to avoid anything damaging the cable sheath.
So the finer the particle size, the better.
"quarry dust" is widely used. It has sharp corners; but the maximum dimension is less than the thickness of the sheath on most u/g cables, so unlikely to cause problems.

[Tiger]

Thank you, Jamie. That is reason why I ask here to see if any one have experiences with this.

Normally Gap 7 are used as bedding material for waste pipes. I just wonder can the materials be used for mains cable as well.

Is it sand or friable soil? Is it free from sharp stone?

If yes, then it's compliant

If not, then it's not

I'm not super familiar with the products but just had a look at what I could online and they do look like they probably meet the definition of sand or friable soil

[Tiger]

Hi AlecK, Thanks for your information. I google "quarry dust" and find Gap 7 is also known as Crusher Dust or Quarry dust.

https://www.captaincompost.co.nz/shop/D ... +Dust.html

So in this way, I would assume gap 7 is widely used as bedding materials.

Thank you, again.

The "AP 7" bit means "all passing 7 mm
So the material has passed through a grille / sieve with holes 7 mm square.
for all "AP grades; some of the particles will have a dimension > the 7 mm nominal size; having slipped through endways.

AP 7 is probably fine for nice round pebbles.
But the "S" means scoria, which has sharp corners - and that won't comply with "free of sharp stone".

When it comes to bedding cables, the point is to avoid anything damaging the cable sheath.
So the finer the particle size, the better.
"quarry dust" is widely used. It has sharp corners; but the maximum dimension is less than the thickness of the sheath on most u/g cables, so unlikely to cause problems.

[AlecK]

What is OK for pipes is not necessarily OK for unenclosed cables.
The purpose of bedding pipes is about maintaining alignment, rather than protecting the pipes from damage.
Whereas for cables, it's about avoiding any sharp / hard bits creating pressure on the cable sheath;
which will eventually cause a breakdown.

Note that if you use a wiring enclosure, you don't need the bedding.

[Tiger]

Hi Aleck,

yes, for good practice, I always use HD conduit for the underground cable. But I have been told by someone that it is common to have unenclosed cables in the bedding of gap 7 which I do not think it is good pratice especially when the cables are buried under driveway.

By the way, Marley provides the continuous power duct which typically used with directional drilling / trenchless installations. But these products do not have duty rating, i.e. there are no information telling they are Heavy duty or Medium duty. I called Marley and the lady who picked up my phone had no idea about this as well. While I saw some of the All-in-one company uses these continuous power duct for mains cable, not sure these guys are applying Category A or Category B underground wiring system. Still waiting for Marley's response on this issue but if you know more information, please let us know.

Thank you again, AlecK. I learnt a lot from you on this forum. Wish I have the chance to meet you and thank you face to face.

What is OK for pipes is not necessarily OK for unenclosed cables.
The purpose of bedding pipes is about maintaining alignment, rather than protecting the pipes from damage.
Whereas for cables, it's about avoiding any sharp / hard bits creating pressure on the cable sheath;
which will eventually cause a breakdown.

Note that if you use a wiring enclosure, you don't need the bedding.

[AlecK]

The cable specs matter also.
Eg N/S rated for direct burial has increased sheath thickness; from memory 3.2 mm.

[Peter]

gap 7 is no where near sand , I would not consider it . Bits of freshly created 7mm long very sharp shards of stone !