Does this component exsist?

[sheriff]

Hi I am an electrician with a basic understanding of electronics. I am looking for a component that will only allow a circuit to flow when a certain amount of current is present.
For example the circuit would remain open until more than 20mA was present. Any help with this would be greatly appreciated

 

[Nigel Goodwin]

Hi I am an electrician with a basic understanding of electronics. I am looking for a component that will only allow a circuit to flow when a certain amount of current is present.
For example the circuit would remain open until more than 20mA was present. Any help with this would be greatly appreciated

No, your question makes no sense, if a circuit is open, then 20mA can't flow.

Assuming you want to close an entirely different circuit when 20mA flows in the first one, then it's fairly easy - but really application specific.

 

[sheriff]

Its tricky for me to explain but what i am trying to do is protect the appliance from test voltages like the insulation resistance test across L and N. This test is carried out at 500v 10mA so i am trying to divert this away from the appliance to avoid damage. but when normal 230v is present i want the circuit to operate as normal

 

[Nigel Goodwin]

Assuming you mean PAT testing?, or using a 'Megger'?, you should know what you should be doing, and what you shouldn't - there's no need or reason for such a device. You also don't do an insulation test between live and neutral, insulation is tested to earth.

 

[sheriff]

The test is for the circuit supplying the device eg a light fitting. it is easy to carry out the test before the fittings are connected but if you come back to carry out a periodic inspection at a later date you are unable to gain readings without disconnecting all the fittings. I am trying to find a way around this problem

 

[Nigel Goodwin]

The test is for the circuit supplying the device eg a light fitting. it is easy to carry out the test before the fittings are connected but if you come back to carry out a periodic inspection at a later date you are unable to gain readings without disconnecting all the fittings. I am trying to find a way around this problem

Sorry, but I'm still baffled as to what your 'problem' is, EXACTLY what are you testing, what are you testing it for, and how are you doing it.

 

[Hero999]

A much lower current is used for PAT, 10mA is much too high.

 

[sheriff]

The test is the insulation resistance of the cables in the circuit. the problem is that items like fans and light fittings can cause duff readings. The only way is to disconnect each item and then test the cables that way. I am trying to find something to go in line to break the circuit before it enters the fitting to prevent having to remove all the lamps or disconnecting the fittings. I think the tester runs the insulation test at 500v 3mA. I was hoping to find a component that would nt allow the test voltage/current through. but would allow the normal running voltage/current through.
so in summary the test is for electrical saftey of the circuit supply the fittings. not the fittings

 

[Hero999]

You shouldn't need to test fans in this manner because it can damage certain components, such as speed controller TRIACs and motor start and RFI suppression capacitors.

If you really want to test the cable then have you tried turning the power switch off?

 

[Nigel Goodwin]

The test is the insulation resistance of the cables in the circuit. the problem is that items like fans and light fittings can cause duff readings. The only way is to disconnect each item and then test the cables that way. I am trying to find something to go in line to break the circuit before it enters the fitting to prevent having to remove all the lamps or disconnecting the fittings. I think the tester runs the insulation test at 500v 3mA. I was hoping to find a component that would nt allow the test voltage/current through. but would allow the normal running voltage/current through.
so in summary the test is for electrical saftey of the circuit supply the fittings. not the fittings

Insulation testers don't 'apply' a certain amount of current, they apply a voltage (500V is usual), and limit any current that might flow to a low value. There's no 'magic' device that can somehow disconnect everything on the off chance someone might be testing the insulation.

However, there shouldn't be anything in a light fitting that leaks to earth anyway, certainly not at only 500V - it's a different matter when PAT testing, as you apply up to 3000V.

 

[sheriff]

It is more for the flouresent light fittings, for example. If i am carrying out a test on a warehouse and all the light fittings are 4m up in the air and there are 10 on each circuit. then i would have to remove all the lamps or disconnect each fitting to gain a satisfactory reading. the problem with this is the time it takes to disconnect the fittings and also there is a chance that you might cause a fault when you reconnect the fittings. i am trying to find a way which would save time but also would nt require any disconnecting of the cables

 

[kizzap]

I think I get what you are on about, but I dont think that it is possible to develop such a circuit...

The problem is that you are trying to compare the current being drawn by the device, with the 20mA, before the device is activated. So you will always be comparing the 20mA current with zero current because the device isnt being allowed to draw power.

It makes me wonder though, if what I am assuming is true, is that you want to test the wires with a quick jolt of electricity (no idea how the testing is working), if it would be possible to use a relay in parallel with the switch, near the appliance, that would only allow the device to turn on when there is a continous circuit attached? No idea if this would work though.

Although I do have to say, I cannot see a reason why this sort of circuit would need to be used. I was under the impression that in testing the cables, you would have to follow some form of strict guidelines to do it properly?

 

[Nigel Goodwin]

It is more for the flouresent light fittings, for example. If i am carrying out a test on a warehouse and all the light fittings are 4m up in the air and there are 10 on each circuit. then i would have to remove all the lamps or disconnect each fitting to gain a satisfactory reading. the problem with this is the time it takes to disconnect the fittings and also there is a chance that you might cause a fault when you reconnect the fittings. i am trying to find a way which would save time but also would nt require any disconnecting of the cables

And all you're doing is testing insulation resistance to earth?, I see no reason to disconnect them at all - just connect Live and Neutral together, and measure the resistance to earth with your megger. The 500V used should be low enough not to cause leakage through the capacitors in the fittings.

 

[olly_k]

Do people test light fittings in this manner?

 

[Nigel Goodwin]

Do people test light fittings in this manner?

I've no idea, it all seems a bit bizzare?.

 

[Hero999]

I know electrical wiring is often tested at 500V in installations.

I think there's little point unless the wiring could be subject to mechanical abrasion or is connected to lots of switches or sockets via terminal screws which could easily work loose.

The only thing I'd bother testing is the earth insulation resistance and maybe I'd temporarily connect a live conductor to earth to check the earth continuity so I know a fuse will blow if it happened when the power is applied.

 

[yorkshire wizard]

You could put in a circuit that disconnect the fittings which is very simple I have done in big factories disconnect l&n at each fitting ?
17 th edi req ins res between l&n, l&e n&e ?

Hope this helps

 

[Hero999]

Testing to the complete 17th edition is not always practical nor is it helpful.

I think he wanted to avoid removing all of the lamps from their fittings.

 

[yorkshire wizard]

Testing to the complete 17th edition is not always practical nor is it helpful.

I think he wanted to avoid removing all of the lamps from their fittings.

Most electrical installations req testing to the 17th and the certs indicate so? and a simple circuit can make it so that lamps and/or fittings would not need disconnecting but of course would the cost be worth it??

 

[Hero999]

I know, I've done it before.

Often parts of the test need to be omitted, for example if there's a lamp dimmer or fluorescent lamp powered by an SMPS the sensitive semiconductors could be damaged by connecting them to 500V.

More often than not, the sensitive parts are isolated and a visual inspection is good enough.

What's the chances of a piece of cable embedded in a wall developing a short circuit between the live conductors?

Next to nil, in reality problems are going to occur in fittings such as junction boxes, lamp holders and switches where insulation can easily be abraded. Flexible cables are most likely to develop faults and they're normally only used for pendant lamps and portable appliances.