Is it much mor awkward working on 3 phase

[riveralt]

The ten point test system was drummed into us at college for the 2391 course.

Kinda implies that they drummed out any real understanding of what you were doing?

Quite the opposite, it re-enforced the importance of safe working whether it is single phase or three phase.
As far as electrical safety, and safe isolation procedures in particular, short cuts are for fools and those who like to test their insurance policies.

 

[JohnW2]

The ten point test system was drummed into us at college for the 2391 course.

Kinda implies that they drummed out any real understanding of what you were doing?

Whilst for 99% of the time I would be campaigning for elimination of 'parrot fashion drumming in', in favour of teaching an understanding of the underlying principles, I think this is one sort of area in which the 'drumming in' is probably justfied. Hopefully most will also learn the underlying principles but a few may not. Furthermore, some redundancy in safety issues as crucial as this does no harm at all, and can occasionally save lives.

Kind Regards, John.

 

[ericmark]

OK you have done the 10 point test and all clear.

Then the contactor closes!!!!

One must know the system your working on.

 

[ban-all-sheds]

short cuts are for fools and those who like to test their insurance policies.

It's GCSE Algebra.

L1 = L2.

L2 = L3.

Does L1 = L3?

Pointless tests where you know the outcome are for the superstitious and OCD sufferers.

 

[JohnW2]

It's GCSE Algebra.
L1 = L2.
L2 = L3.
Does L1 = L3?

Hmmm. As you've written that, the GCSE answer is 'Yes', but the A-Level student would probably realise that we're talking about vector algebra, but have been given magnitudes without phases, so would hopefully answer 'not necessarily' or 'cannot tell'. If you had written L1 = L2 = 0 and L2 = L3 = 0, then that would have been closer to the point you were trying to make. You probably will regard that as nit-picking but, much more important ....

Pointless tests where you know the outcome are for the superstitious and OCD sufferers.

If you know the outcome, then you're obviously correct. However, we're not talking about absolute facts, but of the results of tests undertaken by fallable human beings using fallable machines - and that is particularly relevant if the results one is getting are zero; the easiest way to measure a zero pd is no fail to make proper contact with the circuit being tested.

In a world full of fallabilities, even a carpenter will often measure in two different ways (e.g. from both ends of a piece of timber) before cutting, even though one of the measurements is theoretically redundant. In many safety critical situations, a second person is often required to confirm results, so when working alone redundancy and/or repetition is very wise. In fact, in the real world, a bit of superstition or OCD in such situations can save lives.

Kind Regards, John.

 

[skenk]

Have you never double-checked anything before BAS? Length of worktop to cut? House keys? Ever looked both ways more than once before crossing the road?

Pointless! OCD! blah blah blah

 

[holmslaw]

..

 

[ricicle]

Thats why the 10 point test is the only way to test for dead.

Agreed - for the sake of an extra 5 or 10 seconds work.

 

[bernardgreen]

After testing for dead putting a shorting clamp to ground ( or CPC ) on each conductor reduces the risk if some remote operation could re-energise the circuit.

Removing the wire supplying the contactor's coil or putting an insulating comb between its contacts further reduces risks.

 

[Spark123]

What if a fuse has blown on L2 and is left floating? L1-L2 could show dead, L2-L3 dead, L1-L3 400v
OK - most AVIs don't use that much current so they'll probably show something but the drummond test lamps may fail to illuminate.
Personally I like to stick to what I know and test all combinations.

 

[ban-all-sheds]

What if a fuse has blown on L2 and is left floating? L1-L2 could show dead, L2-L3 dead, L1-L3 400v

I didn't think about that - I was only looking at it from the perspective of a circuit with nothing wrong, no broken conductors, no fuses gone on one phase etc, basically 100% OK, and testing to confirm that the isolation of it had worked.

 

[efiste2]

"Phasing out" or checking phases are the same is probably one of the most important tasks we carry out, when energising new substations, energising new consumers or even syncing on generators......the good old drummond test lamp is your best friend ......

 

[SimonH2]

It's worth pointing out that this is exactly the same single-phase 3-wire arrangement which is used to provide power to homes across North America, just at different voltages, and with the "full" 3-wire supply being the norm even for a regular house.

The standard residential supply in the U.S. & Canada is delivered over 3-wires at 120/240V, and distribution boards have two live busbars. Lighting & general-purpose outlets for all small appliances are 120V, connected between one outer and the neutral, with the load distributed as evenly as possible between the two sides of the board. Large fixed appliances (stove, clothes dryer, larger air-conditioning units & heaters etc.) are fed with 240V by way of a double-pole breaker.

Not only that, but it is my understanding that it's common for a double socket to be fed by a 3-core (plus earth) cable with a shared neutral. Exactly the same saving in copper applies since the neutral carries the difference in current between the loads in the two sockets.

Then I was unable to explain why if L1-L2=0 and L2-L3=0, L1-L3 would not also be 0.

If they were done first, and you knew that L1=L2=L3, you'd only need to do one Ln-E and one Ln-N.

And if they were both 0, N-E would be redundant.

OK, how about L1-L2=0, L2-L3=0, L1-N, L2-N, L3-N all =0, L1-E, L2-E, L3-E all =0, N-E=0

Would you grab hold of L1 and L3 ?

 

[sparkticus]

OK, how about L1-L2=0, L2-L3=0, L1-N, L2-N, L3-N all =0, L1-E, L2-E, L3-E all =0, N-E=0

Would you grab hold of L1 and L3 ?

No! I would use an apprentice for that

But seriously I would not. Without thinking this through in great detail if all isolators where in the closed (on) position and the primary of a delta/star developed an OC primary between - primary L1 and primary L3 essentially leaving one leg inducing current into the secondary then I suspect it is possible to see a PD across secondary of L1 - L3 only. I may be completely wrong and really need to sketch that out but something like that anyway.

EDIT: Would not an open Neutral and open L2 on a star secondary
offer those readings with PD across L1 - L3?

 

[Paul_C]

Not only that, but it is my understanding that it's common for a double socket to be fed by a 3-core (plus earth) cable with a shared neutral. Exactly the same saving in copper applies since the neutral carries the difference in current between the loads in the two sockets.

General-purpose outlets in living areas, bedrooms and so on tend to be wired mostly on straight 2-wire 120V circuits, but yes, a split-wired duplex receptacle on a 3-wire 120/240V branch circuit is common where the outlet is intended to feed two fairly substantial loads (a duplex under a kitchen counter for dishwasher and garbage disposal is a common one).