question time

[cfatdave]

hey my name is cfatdave and ive got a question in an exam next week for electrical installation & its doing my brain in. could you answer this question and show the method please

The current drawn by three 60w fluoresent fitting would be?

 

[imamartian]

ur havin a laff fat dave.... firstly, how do you know the questions in next week's exam !!!!! have you got a crystal ball? and then you want someone else to answer them for you? the idea is that you learn enough to answer the questions !!!

 

[Spark123]

Including a 1.8 factor for discharge lighting and a supply voltage of 230v I get figure of 1.41A
What is the answer meant to be?

 

[ericmark]

I had the same problem for real. I was to install florescent lamps in a series of tunnels and I needed to work out how many I could run on each 16A supply.

Quick calculation approx 1/2 amp each so should be able to have 32 lamps per 16A 110v supply so to be on save side I used 25 lamps per supply.

However shortly after fitting them they tripped out. On testing supply I was drawing around the 20A mark which of course was why they tripped.

I then took a single unit and started to play with it in the workshop. The auto transformer had 110 and 127 volt tapping and changing it to 127 reduced power from 0.8A to 0.6A which should have reduced load to 15A.

Again however not according to plan last few lamps would not start. So the last 5 lamps were set to 110 volt on auto transformer and first 20 set to 127 volt. At this the whole string ran at 16A.

From this it can be seen not as simple as dividing watts by volts to get amps. The lamp it seemed from tests would work on less than 60W but was likely not to start and once one reached the running voltage a very small increase would produce an out of proportion increase in current used. Also removing a capacitor could increase it from 0.6 to 1.2 amp so considering a capacitor could fail it would seem with an inductive control system one needs to allow double the ratted wattage.

However today we have HF florescent fittings and these are the reverse as far as current increase as voltage increases. So with HF units much less leeway is required.

I was in a way lucky with 110 volt system to be able to slightly vary the voltage with a 230 volt system there is little or no control.

So when answering a question like you have presented one would need to answer with a range. So for 3 x 60W units between 0.8 and 1.3 amp at 230 volt depending on type of control gear and ratted voltage of the lamps.

As a result it would seem an unlikely question in an exam as one would want to have a fixed answer.

I do find with courses I am doing there is a problem where ones level is above that for most students taking the exam. One has to try to work out what is expected. Maths is good where it will ask for a real answer so one knows one is not expected to use imaginary numbers. However the Physics is very badly written and one has to guess the level and assume as one has not been taught calculus in the class one is not expected to use it to find the area under a graph.

You must judge yourself to what level you are expected to answer. On a level 3 exam I would expect you would need to refer to inrush but as to if you need to refer to the failings of the inductance control gear I don't know.

 

[TicklyT]

Whatever you think the answer should be, provide your calculations to prove your argument.

With the information you have provided, the answer, of course, is zero. That answer is likely to change once you take the lights out of their packaging and connect them to your unspecified power source.

Do you want to show the power consumption for the lamps under steady running conditions, or do you want to illustrate a method to select suitable cabling and protective devices for the installation?

Do you wish to correct your answer for power factor, starting inrush currents, supply voltage etc. etc?