Contactors and relays

[Electron1]

Hi all,I am new to the trade. I am studying at collage and work as an apprentice!

Could some one please help me with how contactors and relays work!

What are they used for?

What's there benefits?

When would u use them?

 

[M107]

To switch heavy or multiple loads via a lower rated switching circuit.

Cost saving on switch gear & cable

Banks of florries/heating/roller doors/timer circuits.

 

[Electron1]

Can u tell me more please! When for example would u use it for lighting? How would a contactor be wired?

?

 

[M107]

Have you seen a contactor? Well they come in varying sizes & poles

http://www.tlc-direct.co.uk/Images/Products/size_3/WYMESB20.JPG

http://www.tlc-direct.co.uk/Images/Products/size_3/WYMESB40.JPG

They act as a switch turning on large loads (look on the front of the units & you'll see a simple diagram), but the switch (coil of the contactor, terminals A1 & A2) is triggered by a much lower rated circuit..

So you could have for arguments sake 100 highbays lights wired in 10mm circuit cable through the contactor, but the contactor coil is operated by a small 3 amp switch circuit.

Or if you know how the coil could be switched using a logic driven voltage;

Our older stuff is a BT signal-logic triggered 24v pulse to elv relay - pulses 230v to latch in a tematron timer - timer triggers contactor coils for all lighting in the building, bay door opening, sounders.

The new kit is BT signal-logic triggered 24v pulse to 24v actuated din mounted timer - switches 230v 5amp circuit to trigger contactor coils for all lighting in the building, bay door opening, sounders........

Canoe is the panel man & could probably go further with that type of use...

 

[Evans Electric]

Also used for motors , Electron, you may have covered in college that if , say there is power loss , motor driven devices/ machines must not re-start on their own for safety reasons.

Therefore they are fed via a contactor (Starter) the coil holds the contactor closed but if theres a power cut or a stop button is pushed , the contactor drops out and won't re-start until someone does it.

Have a look at one and all will become clear !

And , basically , a contactor switches higher loads ,say a heating load switched by a time switch.

A relay is for switching and change over control circuits , they usually have a number of normally closed contacts, normally open and change over ones .

Again have a look at one .

 

[Electron1]

Ok I think I get the basics! So really contractors are used if u need to control something with a higher load then the switch ect, eg if u have a 6amp lighting cct with a load of 20amp, u would use the contactor to give u a higher rating in order to control the lights, even though it's supplied via a 6amp mcb??

 

[Electron1]

Yes please mate! So why not just use a 20amp dp switch to turn them on or off?

I have seen a consumer unit once extended with contactors? They were in a box then went into the consumer unit?

Can it be done that way!

The only reason I am asking is because or boardem lol and I seen a post about it and didn't understand so u thought I would learn hope that's ok?

 

[ProDave]

I used a contactor recently where a frost thermostat had to switch on half a dozen electric heaters which would have been far too much load for the contact rating of the frost thermostat.

And just to make it interesting, some, but not all of the heaters needed a manual switch to turn them on when needed.

And just a week ago I was fault finding on a whole lot of outside lights on a commercial building. Here a whole lot of sodium lights around the building, and lamp posts in the car park were all switched from one disk sensor. Again the total load far exceeded the switching rating of the sensor, so 2 contactors were used to switch all the lights.

 

[NozSpark]

Here's one of mine,,, not quite as bi as Canoes, but it's domestic

It's a granny annex, the main house has E7 heating, but I couldn't export the E7 supply....

Before Steptoe steps in it's TNS

.....so I exported the normal tarrif supply and switched the storage heaters with a din rail timeclock and contactor (bottom right of CU)

 

[NozSpark]

Cheers mate,,,

Even though it is Proteus

It was significantly cheaper than any other brands (and I mean >

 

[Electron1]

Can u explain what looks like a 2.5mm incoming into the conactor then 25mm out in to the main switch? What was the need for the contactor in this situation?

 

[NozSpark]

OK, here's another picture (covers on), it may help a little

Right, the reason I did it this way is because I couldn't get an E7 supply over to this board,,, the supply cables were existing 10mm 4core SWA,,, I remember now the supply back at the house was TNC-S and I didn't want to export the earth,, although the SWA armour is earthed at the house I isoated it in trunking beside the CU's (heatshrink IIRC) and stuck a rod in...

Now the std rate supply does go E7 when the teleswitch changes over, and I needed a way to switch the heating (E7) on to charge,,,, they are combi heaters with a fan heater available during the day to top up, that's why there are 2 sets of heater MCBs

Back to the wiring specifically.. The timeclock is fed from the RCBO (remember it's a TT now) and the switched output goes to A1 of the contactor, A2 is connected to the RCBO neutral. The supply comes in to the contactor at the bottom and te switched side (top) supplies the RCD to it's left which in turn feeds the E7 MCB's

I hope that makes sense

 

[kerching]

Think of it like the ignition switch in a car.

Flimsy little switch wired in a bit of 2.5 ish cable BUT it switches the starter motor current which can be [at a guess] 80/100A

So the ignition switch energises the coil. This creates a magnetic field which pulls in the armature of the relay [pi44 ALL CURRENT], this in turns shorts out the contacts for the high starting current. Well it made sense to me and IT IS Very late........... :coat

 

[Evans Electric]

Yes please mate! So why not just use a 20amp dp switch to turn them on or off? I have seen a consumer unit once extended with contactors? They were in a box then went into the consumer unit?

Can it be done that way!

The only reason I am asking is because or boardem lol and I seen a post about it and didn't understand so u thought I would learn hope that's ok?

Electron , the contacts of the contactor could be switching three different circuits /and /or phases or , say, three rows of lighting trunking .

Or imagine a key switch in a school metalwork room, teacher has the key which feeds the coil of a contactor, the poles of the contactor feed all the machines and plugs in the room , for safety reasons.

 

[steptoe]

OK, here's another picture (covers on), it may help a little

Right, the reason I did it this way is because I couldn't get an E7 supply over to this board,,, the supply cables were existing 10mm 4core SWA,,, I remember now the supply back at the house was TNC-S and I didn't want to export the earth,, although the SWA armour is earthed at the house I isoated it in trunking beside the CU's (heatshrink IIRC) and stuck a rod in...

Now the std rate supply does go E7 when the teleswitch changes over, and I needed a way to switch the heating (E7) on to charge,,,, they are combi heaters with a fan heater available during the day to top up, that's why there are 2 sets of heater MCBs

Back to the wiring specifically.. The timeclock is fed from the RCBO (remember it's a TT now ) and the switched output goes to A1 of the contactor, A2 is connected to the RCBO neutral. The supply comes in to the contactor at the bottom and te switched side (top) supplies the RCD to it's left which in turn feeds the E7 MCB's

I hope that makes sense

deviation alert

I will start a new thread on this tomorrow,

but this raises a very good question?

does Noz actually require an RCBO here?

if he does should it be DP ?

should it be 100mA to allow discrimination with the RCD ?

should it be TD or S to allow discrimination with the RCD ?

should he have a 100mA front end for this CU ?

sorry to hijack this thread,

I would transfer NOZs pics and things over to a new thread, but apparently Brians busy so cant be bothered trying to help me. :|

if someone else fancies starting this in the student bit it might be an idea,

thanks,

Albert.

 

[NozSpark]

But I know what I've done

If you want to use the photos in another thread then go ahead,,, just copy what's between the [ img] tags

I guess you don't really want my input, in the begining?

 

[steptoe]

not really,

but I think it may make a very good discussion point in the student section,

as to why you have done what you have, and the reasoning behind it.

something slightly off normal shall we say, but still, within a domestic enviroment.

Noz, why dont you just start it anyway?

you have the pics.

steal my questions if you want,

or better still, ask some better ones.

 

[Electron1]

So could u for example have 10 motors say max load is 30A,

a 2.5mm feeding a 20A dp switch and then into a contactor rated at 30A! The mcb is also rated at 32A! I don't see how the 2.5 cable will be big enuf to carry the load

 

[NozSpark]

I could if you like, but I'd only nick your questions TBH as I might be a bit close to the job IYKWIM

Anyway,, off to bed now

 

[steptoe]

electron,

I think the easy way is to describe the contactor as a switch,

and you use a smaller switch to tell the bigger switch(the contactor) to turn on,

the smaller switch uses a smaller cable, a 2.5 as it can only switch 20 amps,(it could really be much much smaller, 1 amp even!), when you turn the little switch on it carries a current to the large switch(the contactor),

the contactor has a large cable running to it, maybe 25mm, as it can switch 60amps,

the little switch energises a coil inside the large switch which in turn pulls a winding in the big switch across to make contact, now the big switch has been turned on,

so you have been able to use a little 3amp switch to turn on a load of 60amps, simply by using a contactor.

its getting late, does any of that make sense to anyone?