Please help make my life a touch easier....

Discussion in 'The Projects Forum' started by ianhager, Sep 3, 2011.

  1. ianhager

    Thread Starter New Member

    Sep 3, 2011

    I'd really appreciate some help with this. :)

    My job is to perform the periodic inspection and testing of electrical installations in line with BS7671. Sometimes I have to trace unknown circuits and I do this by plugging in something like a toaster or kettle to induce a load on to the circuit, then would use a clamp meter to locate the load in the distribution boad. As utilising a kettle or toaster in this way isn't practical, i'd like help designing something which would probably resemble a 13 amp plug top on a flex with the flex running into a small plastic adaptable box. Inside the box (this is where you come in) ...... what would i need to replicate a load, maybe 10 amps so easily identifiable at the board. ie, transformer? resistors?

    Thanks for taking the time out to read this and also in anticipation for any help given.

  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    A 10amp load is a 10A load to a clamp-on. You can't really simulate it. 10A at 240VAC is 2.4 KW. That's a lot of heat to dissipate...and won't happen in "a small plastic adaptable box". If you are just tracing wires, there are commercial electronic circuit tracing devices. Connect the "small" signal generator box to an outlet or breaker panel and use the sensor probe to locate outlets or breakers that are on that circuit.

    Google: electrical circuit tracer

  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Why not build a resistor load bank?

    You will need a fan to remove the generated heat, resistors and a box made out of perforated metal sheets for example (which you should connect to earth when you use it.

    If you want 10A @120V you need to dissipate 1200W in resistors and the resistors need to have 12 Ohms in total.
    Just put whatever resistors is easily available to you in series or parallel to give you the total resistance needed and the total power plus a safety margin.

    example: 10x1.2R in series each having 150W. Any other combination is possible with other values... Doesn't have to be exactly 10A neither, right...
  4. Arius007

    New Member

    Sep 2, 2011

    Equipment for this purpose is commercially available. Here's one example:

    I used to use equipment designed for telecoms for this task. It comes in two parts - very similar to the one above and not as expensive. The signal generator comes with crocodile clips. Similar to this:

    Both versions work by injecting a signal and the detector lets you hear the signal as you trace the wiring.
  5. SgtWookie


    Jul 17, 2007
    Hello Ian, and welcome to the Forums.

    Well, thank you very kindly for that reference. In the years since I've been a member on here, you are the very first that I have seen make reference to any sort of written electrical standard on that side of the pond!
    Just for everyone elses' enlightment:
    which lead me to the international standard:
    so that reference has been very useful indeed.

    The US and the UK are two great countries separated by an ocean and a common language. ;) I'd never heard reference to a "plug top" either, believe it or not. In the US, we simply refer to them as a "plug".

    Do your plug tops have flat blades that are inline plus an earth/ground terminal, like this one?


    So, I take "plug top on a flex" meaning an electrical cord assembly with a plug suitable for a typical 13A UK wall outlet, which I've heard is nominally 220VAC @ 50Hz.

    220V*10A=2,200 Watts - well then, here's the rub - how does one "get rid of", aka dissipate, that much power? A purely resistive load will get rid of the power by converting it into heat; a lot of heat. Transformers are rather efficient at AC power conversion, but one rated for ~2,200VA will be rather large and heavy - and you would still need some sort of load to get rid of the power.

    Have you considered using one of those ceramic heaters with a fan inside? They are generally quite small and lightweight, and cool off in a hurry when not in use. That would likely be more convenient than a toaster, kettle or hotplate; as the latter would require a significant cool-down time before packing up.

    Sort of "thinking out loud" here...
    Aside from simply burning off power in a load, you might consider placing a low-voltage frequency (say, 1kHz to 10kHz; something in the audible range) on the mains line instead, coupled using a transformer. That would make identification of the "hot" wire fairly easy if the breakers/fuses are off/out/disconnected, and could be battery powered for installations that don't have power. However, identification of the neutral line would be more difficult, as the signal would be attenuated very significantly at the distribution panel due to being connected to the mains/earth.
  6. SgtWookie


    Jul 17, 2007
    Gee, I seem to be quite slow this AM... ;) The device that Arius007 is on the lines of what I was thinking; but it will likely have a similar deficiency in that the neutral line will be difficult to locate.

    Now that I'm thinking more about it, a steady 10A load could be hard to isolate if the building is in use; as people might be using various appliances. It could be much more useful if the load were being cycled on and off every couple of seconds; you would see the fluctuations in your ammeter. Also, that would enable you to use a much smaller load, yet still identify the circuit.
  7. Arius007

    New Member

    Sep 2, 2011
    Often the neutral can be identified because it's carried in the same cable as the phase/live conductor, so once you've identified the live it's a matter of tracing back to the cable and identifying the neutral from there. Obviously this isn't always the case and it's not something you ideally want to be doing on a live distribution board.
  8. SgtWookie


    Jul 17, 2007
    Agreed. Here in the States, wiring is typically performed using "Romex" for the last 50 years or so:

    The plastic cover is usually white or yellow. The neutral wire is white, black is L1/L2 aka "hot", and the earth/ground wire is typically paper-wrapped; the wrapping is removed when the sheath is stripped.

    However, one sometimes runs into installations that look like this:


    which makes it quite difficult to sort which goes with which. By the way, the above installation violates the National Electrical Code here in the US; it would have to be completely re-done.
  9. someonesdad

    Senior Member

    Jul 7, 2009
    One of the advantages of testing as you describe with a 10 A load is that you can also validate that the circuit is capable of carrying a decent current. I concur with Wookie that just about the best load would be one of those small ceramic heaters. Building something yourself to dissipate 2 to 3 kW of power is not a trivial task because, for safety, you have to design it to be capable of being left plugged in and turned on. Unless there's a reliable method for shutting things down when things get too hot, you would have a problem. Someone has already done that design work for the heater. There's nothing overly sophisticated about the design, but why do the work when you can buy it so cheaply?

    If you just want to identify circuits and don't care about seeing them also operate under load, there are many different circuit tracers available, as Ken and others mentioned. They operate by injecting a signal on a wire, then detecting that signal. My Gardner-Bender unit uses a 36 V peak-to-peak 2 kHz signal that is frequency modulated; the detector warbles because of this FM. However, they can be fooled by capacitive coupling; your method of putting a 10 A load on a circuit is reliable and can only be fooled by a simultaneous identical current on another circuit.

    Here's another method that comes to mind (this is theoretical; I haven't tried it). Fluke makes a clamp on ammeter used to detect leakage currents that can measure leakage currents down into the μA range (but it's not cheap -- around $600). You could make a simple device that would cause a modulated leakage current (on/off, say at 0.5 Hz or so) that's not large enough to trip any GFI (residual current) devices -- say a mA or two. Just plug it into an outlet, then monitor the wires for that modulated current. An advantage over the circuit tracers is that it couldn't be fooled by capacitive coupling. The circuit would be simple to build -- a suitable MOSFET driven by a 555 with an appropriate resistor to get the desired leakage current.
    Last edited: Sep 3, 2011