Is this true?  I need help with this please - Page 1
Forum section: electronics / general

author: melrel
author_rank: Contributor
timestamp: February 03, 2013, 05:52:15 pm
content: I have been following a project (not of my own) where LED outside lights were purchased to illuminate a pathway about 50 metres long. The LEDs are contained in a weatherproof housing. The idea was to bury the lights in holes dug in the ground with mains voltage supplied by armoured cable. The lights were duly installed by registered electricians and all was well. Until that is, when the builder arrived and layed concrete flagstones over the cable and lights. Holes had been drilled in the flags to allow the LEDs to be flush mounted. The builder laid the flags OK, but in concrete. The armoured cable became encased in the set concrete. (Yeh I thought the cable should have been buried but it wasn't) When all the flags were laid the lights wouldn't work after a few days. Now here's my question......... the electricians said that the concrete had crushed the armoured cable when it set and caused a voltage drop which the in-line circuit breaker had picked up and closed down the mains supply. The cable had not been pierced in any way. Is this true guys? If you crush a cable does does it cause a voltage drop?

author: SeanB
author_rank: Super Contributor
timestamp: February 03, 2013, 06:12:15 pm
content: More likely the vibrators used to remove the air have torn a cable out of one of the boxes, or torn the gland out, and water got in. Concrete actually shrinks when curing, which will not crush a cable, they put plastic piping in concrete, and those are so thin that they bend under their own weight. they survive being cast, and an armoured cable does as well.

author: melrel
author_rank: Contributor
timestamp: February 03, 2013, 06:24:18 pm
content: Thanks for the comment SeanB but.......the concrete wasn't vibrated it was laid by hand with a trowel!!! Also the cable wasn't encased in anything - just laid on top of the ground and that's why it became part of the concrete.

author: SeanB
author_rank: Super Contributor
timestamp: February 03, 2013, 06:30:00 pm
content: Then you got water in through a gland or a box that is not sealed correctly. Armoured cable survives having a bulldozer pull it up out of the ground to a point.

author: IanB
author_rank: Super Contributor
timestamp: February 03, 2013, 08:14:58 pm
content: Wet setting concrete is very corrosive. Imagine leaky alkaline batteries only much worse. I have no experience with this, but I would imagine that any metal parts that are going to come in contact with concrete should be specified for that environment. If there were any exposed metal parts like glands or fittings they might be in danger. It would be safest if all parts coming into contact with concrete are plastic.

author: IvoS
author_rank: Frequent Contributor
timestamp: February 03, 2013, 08:29:23 pm
content: Here in States such installation would be illegal since MC (metal clad) cable is not allowed to be encased in concrete nor is designed for direct burial. EMT (electrical metallic tubing), rigid pipe or PVC pipe can be used for encased concrete, then you pull wires into it.

author: IanB
author_rank: Super Contributor
timestamp: February 03, 2013, 08:49:47 pm
content: There are designs of armoured cable where there is a PVC outer sheath. Cable like that could be buried as long as the fittings were suitable for the environment. The OP does not give enough detail for us to know exactly what the circumstances are or what kind of cable was used.One point of note here is that a project should have a main contractor who coordinates all the pieces that have to come together. If you independently contract one part to an electrician and another part to a builder without being in control of what happens at the interface where the electrical work and the building work meet you could be setting yourself up for trouble.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 03, 2013, 10:58:17 pm
content: armoured cable is... armoured, like a military tank. it is unlikely that it got damaged during laying down concrete, its also designed to take harsher condition such as corrosive soil, it got strong rubber on the outer part alright. rubber-thick wires shield-rubber-conductors. 50 meters isnt long, maybe you can do continuity check or earth leakage check? it is possible though there is discontinuity in the conductors due to manufacturing defect, but i'm not sure not an expert.if you have few different contractors doing the same project, they tend to fault each other. when one person made a mistake, he will tell its the other contractor making the fault. you also need to provide more details (pictures) on how the installation is made. someone drilled right in the middle of the cable core (after the concrete is set) damaging the conductors is also possible.

author: melrel
author_rank: Contributor
timestamp: February 03, 2013, 11:55:00 pm
content: Thanks to all you guys for your comments. This is EXACTLY what happened IanB. No project leader. Both sides did what they thought best. Also IanB you were right in your assumption that the armoured cable had a plastic sheath. I take your points too Mechtrommer. The armoured cable wasn't damaged but in the end it had to be dug out of the concrete and almost all the entire length replaced.