I MUST start by reminding everyone that I am not in charge of the SANS Standards Committee and so sometimes the advice I give is dead wrong.
Having said this, the following are answers to questions that have been posed to me over the last few months:
Working on live equipment: You should never ever work on live equipment. Note, I did not say "unless you have to" or "unless the foreman or engineer says so". Never, repeat never. It is a practice that has crept into the industry.
The boss or the production manager says, "Hey, look, we can't have a shut down just to put in a circuit breaker... can't you do it live?"
Well, yes, most of the time you can do it live. Most of the time you won't be in any danger. But not all the time, trust me. If you have a family or hope to have a family, for their sake, OIE - open, isolate and earth - before working on equipment. Even if you don't have a family. Just don't work on live equipment.
Which bring us to earthing:
The other day some electricians were working on a panel. I asked if it was live.
"No," they said, "it is not."
So I asked, "Have you earthed the busbars?"
"Nope," they replied, "we have not."
"Why not?" I asked.
"Oh," they said, "the feeder circuit breaker has a big sign on it that says, ‘Do Not Switch On' and if somebody did switch on there would be a big bang and we'd all be hurt."
I replied: "Oh, so if you touch the bars there will also be a bang, just smaller?"
Hello? Hello?
Many years ago, a foreman electrician told me always to apply working earths and the principle, ‘The Earth You Can See Is
The Earth You Can Trust'. And you know what? The only time I ever got an electrical shock was when somebody else told me that this was done... when it wasn't.
Moving onto other things. You decide to reticulate an open plan office. So, you run trunking in the ceiling void and put 16A socket outlets at intervals. You then plug power poles into the ceiling sockets and use the power poles to feed a set of socket outlets mounted under a desk (say four desks).
Is this legal? Is this a good idea?
In SANS 10142 it states: "6.15.2.2 - The anticipated load of a circuit that feeds socket-outlets shall not exceed 5 kW".
So, on that basis, the system is bad. Oh no, they may argue, since the desk will only feed a computer and printer and so on.
Will it?
Well, until the A/C packs up and the desk owner plugs a heater in. Forget about the whole idea - notwithstanding interpretation of rules, the whole installation is no different in form from a lot of illegally-connected shacks fed by extension cords connected to one socket outlet.
Moving on. A distribution board feeds a sub-distribution board. The sub DB has a 60A isolator, which is fed from a 60A circuit breaker in the distribution board. Is it a better idea to have the sub DB isolator replaced with a circuit breaker?
Well, yes. With an isolator, the 60A feeder circuit breaker will trip for both a fault in the cable and a fault in the sub DB. With the isolator replaced by a circuit breaker it will then be possible to distinguish between a cable fault or a fault in the sub DB - provided the two circuit breakers are suitably chosen to discriminate between each other.
To do this, you need to do as follows: A cable fault will generally always be a short circuit. So if the load is 60A, you choose the feeder circuit breaker to trip slower and at a higher level than the sub DB incoming circuit breaker. In the above example,
I would specify an 80A supply circuit breaker and a 60A incomer. The 60A breaker will then trip on overload while the 80A will trip on cable short-circuit. Geddit?
Oh, I do hope so.
And if I am wrong, please tell me.
machoy@iafrica.com
