No earth core ..but sheath?

[IP4X]

Hi there
Just a quick question please. I am busy doing an inspection and test report for an old commercial building. Think its old NHS sugery.
Now:
There are two 32A socket rings that have used what looks like old fire alarm cable. Its two core (both cores black) cable and I am assuming that there is a metal sheath that acts as the earth core.
I haven't tested yet but was hoping someone has seen this before and could give me some advice. It may be common but I have not seen it before.
Also there is the fact that the earth is not connected to the earth bar in the CU but rather taken from the metal clad CU from the gland..
Any hints, tips, advice, opinions most welcome
Cheers

 

[MNR]

You sure it is not mineral insulated?

 

[Sintra]

Definately sounds like a MICC (pyro) if this is the case then it is perfectly acceptible proveded you are getting earth continuity throughout the circuit. Note back boxes must have a fly lead in this instance.

 

[chris gray]

why would a back box require a link but not at the board?
just curious..

 

[IP4X]

Yip.. I think it is. Cheers for that.
So am I assuming correctly that I dont have to make special mention of this in the report if all the tests are ok? besides the usual wiring codes etc.
cheers

 

[mogga]

Im assuming your an electriian???

 

[IP4X]

Im assuming your an electriian???

Yip..
Actually an Electrical engineer turned electrician. Couldnt see why I spent 4 years at University and earned less than electricians. So did the 16th...17th and 2391.
I have just spent most of my life on MV switchgear and transformers.
But thanks for that... Always nice to be ridiculed for asking honest questions.

 

[andyb]

There is nothing wrong with using 2 core MI, it never was very common which is why you would not of come across it before. In the circuit cpc csa box just put MI, as Sintra says the sockets must have a fly lead to the back box.
It is good practice to have a fly lead at the consumer unit as well but not technically necessary as there is a solid connection between the mi gland and the earth terminal, not so at the sockets.

 

[mogga]

it wasnt the question that made me ask it was the way the question was asked sorry for any offence caused

 

[UNG]

There is nothing wrong with using 2 core MI, it never was very common which is why you would not of come across it before.

I suppose it depends on the type of installations you have worked on. It was very common in NHS property, churches and in the commercial and industrial sectors only cost killed off the use of it in my opinion as it is far superior to firetuf for fire alarms

A number of the high rise and poured concrete buildings built in the 1960's & 70's were wired using the pre terminated MI "octopus system", a friend of my mum worked for BICC and she terminated the pyro on piece work 1 end every 2 - 3 minutes for them can't remember what they were paid per end. BICC produced a video of it don't know if it is on the web somewhere I saw when I was at college many years ago

The only problem with MI is the aluminium variants the alu sheath and cores was and still is if you find any horrible stuff, the AlCUmic alu sheath with copper cores was a bit better but earthing with the sheath was not good due to oxidisation and corrosion at the glands

 

[IP4X]

Cheers guys.. great help.
Tested ok today and I was very surprised by the good readings.
But I suppose irrelevant with a Ze of 608 ohms!!!

 

[Guest123]

Ahhh, I loves MI I does.

I'd use it for everything if I could.

 

[Sam_Butler]

The building I'm maintaining was built in 1925 and the "original" wiring is certainly 1960s or older. Though it's a bit of a patchwork, the lights in the stairwell are bare MI runs (no PVC sheath, but everything is painted over). Demolishing it is actually quite hard. I've heard a sparky joke that that even if a building burnt to the ground the Pyro would probably still be in place!

 

[Andyfm]

They told us at college that it would take a few hits with a hammer as well