Fluorescent fittings and power factor.

Discussion in 'General Electronics Chat' started by lightingman, Jun 19, 2013.

  1. lightingman

    Thread Starter Senior Member

    Apr 19, 2007
    Question: Does a four-tube fluorescent fitting consume less power if three of the tubes are removed?

    Answer: No....

    The place where I moved into was full of the office type fluorescent fittings (the type that go into suspended ceilings). They are made by Fitzgerald.There were both 2 foot and 4 foot types.

    With so many units burning away, I decided to remove three tubes from each, to keep the electricity bill down. We are only doing building work, so that was fine.

    After a few months, we got our first electricity bill of £969.00. I thought that it was a little high, so I questioned it. A check revealed that the meter was accurate, so I decided to find the problem.

    I fitted a Gossen Merawat A2000 energy meter/logger to record our usage. It was whilst experimenting with the readings on the PC in the office when I turned on the lights (two of the 4 foot units with only one tube in each). I was shocked to see an increase in current on the blue phase of over 2 amps.
    I thought that I would try adding more tubes and with each tube added, the current decreased until when they were all in, there was a reading of 1.2 amps.

    I then tried it with the lights in the foyer area (12 of the 2 foot units). With just four tubes on (two in one unit and two in another), just over 2.1 Amps. With all tubes in (48 in all) and a reading of 2.6 Amps.

    I can only put this down to the PF capacitors being calculated for all four tubes.

    Any thoughts?
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Yes dont remove the tubes as the Power Factor cap is designed for that purpose, removing tubes alters the PF to Leading, so it will increase current. Ideally buy fittings that give lower current demand and higher Lux output.

    Last edited: Jun 19, 2013
  3. richard.cs


    Mar 3, 2012
    Most likely removing the tubes does save real power, but aparrent power increases. What you are seeing is that with fewer tubes the current through the PFC capacitors is not balanced by the reactive part of the tube current due to the magnetic ballasts. Current goes up but should be out of phase with the voltage so this doesn't correspond to real power consumed.

    Thank about it, if you remove tubes and the real power really did go up then something in the fitting would be getting very hot. Assuming that isn't happening then the increase is reactive power only.

    So it really depends, are you billed for reactive power? Domestic customers aren't, some commercial customers are.