Wiring Brain Teaser

[SparkyChick]

This is another real life problem that I fixed this week and it lead onto quite a lengthy chat in the van about the components used, how they work etc. so I figured I'd post it for you guys to have a punt at.

The problem....

If any of you know Cardiff, you'll know all the apartments etc. that have gone up around the bay. Quality buildings It seems that some of them include air handling units that are running 24/7 providing forced ventilation. These units have, if you will, an overdrive mode that is engaged under certain conditions to greatly increase the air flow rates.

In the apartment in question, overdrive mode is triggered by the bathroom light, the en-suite light and the kitchen light. Unfortunately the 'controller' for this appears to be a pile of poop, with inadequately rated components that are failing, resulting in weird behaviour (the fan stuttering between normal and overdrive) and/or not going into overdrive at all.

The switching is handled by the 'controller' located in a plastic adaptable box by the consumer unit. It has 5 cables coming into it.

3 x 1.5mm single and earth - These provide a switched live supply from the 3 lights that trigger overdrive
1 x 1.5mm twin and earth - This provides the supply for the fan
1 x 1.5mm three core and earth - This provides the connection to the fan (Brown = Permanent Live, Grey = Neutral, Black = Switched live for overdrive mode)

For clarity, the lights are on a different MCB to the fan, but neither have any RCD protection. What the current controller consists of is largely irrelevant, just know that when one of the lights is turned on, it supplies 230v to the overdrive conductor in the 3C+E cable.

Have a punt at drawing up a wiring diagram that achieves the desired results with no unintended consequences

 

[Loki]

Hi @SparkyChick ,


Ok so here's my attempt at your latest brain teaser.

Spoiler

 

[SparkyChick]

Hi @SparkyChick ,

Ok so here's my attempt at your latest brain teaser.

Nice diagram... but think about the very last paragraph of my post... "that achieves the desired result with no unintended consequences", it's very important.

 

[Loki]

Nice diagram... but think about the very last paragraph of my post... "that achieves the desired result with no unintended consequences", it's very important.

Spoiler

Oh yeah ive just realised in my drawing if any one of the lights is on the switch wire will feed the other lights & they will turn on also lol

 

[SparkyChick]

When these things are going on, please put your comments/thoughts/questions etc. in a spoiler so people can choose to read it... or not

 

[Lucien Nunes]

Spoiler: It's a bit cheeky

Not having a neutral for the three trigger inputs that is electrically separate from the fan neutral. I know the current only needs to be tiny, but it shouldn't borrow the neutral by design, surely?

 

[SparkyChick]

Not having a neutral for the three trigger inputs that is electrically separate from the fan neutral. I know the current only needs to be tiny, but it shouldn't borrow the neutral by design, surely?

I wouldn't call it a cheeky spoiler... valid point is a better description, and one that, I'll be honest, didn't really cross my mind at the time.

Both the lighting and the fan are supplied by MCBs on a non-RCD portion of the board, and I only replicated with contactors what a PCB with relays was already doing, but yes it's not an ideal situation really.

I suspect it wouldn't be too much of a problem to put it right as the CU isn't too far away (read inches).

Something for me to consider more carefully the next time something like this comes up. Thanks for pointing out my error

 

[Jay 1987]

Here’s my attempt...

 

[Jay 1987]

The

Here’s my attempt...

the things in the middle are contactors (normally open) which need 230v supply

 

[Lucien Nunes]

Good so far, but there are some connections missing from the contactors. If you replace the anonymous rectangles with the standard symbols for coil and contacts, this will be immediately obvious.

You obviously have a clear grasp of what you want to achieve with the drawing and it conveys the information well, so I'd urge you to move to the next level, using correct circuit symbols and putting away the multicoloured pens .

 

[Jay 1987]

Good so far, but there are some connections missing from the contactors. If you replace the anonymous rectangles with the standard symbols for coil and contacts, this will be immediately obvious.

You obviously have a clear grasp of what you want to achieve with the drawing and it conveys the information well, so I'd urge you to move to the next level, using correct circuit symbols and putting away the multicoloured pens .

Good so far, but there are some connections missing from the contactors. If you replace the anonymous rectangles with the standard symbols for coil and contacts, this will be immediately obvious.

You obviously have a clear grasp of what you want to achieve with the drawing and it conveys the information well, so I'd urge you to move to the next level, using correct circuit symbols and putting away the multicoloured pens .

That took me 3 hours to work out lol seriously.....

 

[davesparks]

Here’s my attempt...

You've got the right idea, and I'm pretty sure you are on the right track to a suitable solution.

However you have created what is known as a borrowed neutral here, this is a very dangerous thing to do.
Your coil connections take a live from one circuit and a neutral from a different circuit. You must always connect to the correct neutral for the circuit you have used the live from.

I know this may sound abrupt or a bit harsh, and I don't want to put you off, but borrowed neutrals are very dangerous.

 

[Lucien Nunes]

True enough; however, according to SparkyChick's description that Jay will have followed, it was wired like that with the borrowed neutral, as I also mentioned above. So the original installers and makers of the relay gubbins are to blame for it!

 

[SparkyChick]

True enough; however, according to SparkyChick's description that Jay will have followed, it was wired like that with the borrowed neutral, as I also mentioned above. So the original installers and makers of the relay gubbins are to blame for it!

Absolutely, but I should have picked up on it and corrected it. Mistake on my part.

 

[Lister1987]

Absolutely, but I should have picked up on it and corrected it. Mistake on my part.

Play it cool, it was intentional and you were testing everyone.....right? ?

 

[Jay 1987]

Here’s my second attempt.....

No rectangles
No coloured pens

hahahah

 

[davesparks]

Here’s my second attempt.....

No rectangles
No coloured pens

hahahah

It's a good clear diagram which I find pretty easy to read.
But it may not work exactly as you are expecting it to.

If you were to turn on all three lights by their switches, and then turn off light 2 only by its switch, what will happen?
It may help to sketch what happens on the diagram with a pencil,

 

[Lucien Nunes]

Much more professional... but wrong!

Problem 1: Exactly as DS says. It's to do with where you are getting the feed to the contactor terminal 1's from.

Problem 2: The fan is supposed to drop back to low speed when none of the lights / contactors are on. But you have done something that will prevent this working...

You're close.

 

[Jay 1987]

Much more professional... but wrong!

Problem 1: Exactly as DS says. It's to do with where you are getting the feed to the contactor terminal 1's from.

Problem 2: The fan is supposed to drop back to low speed when none of the lights / contactors are on. But you have done something that will prevent this working...

You're close.

You make me laugh lucien lol

I don’t know where to go from here but I’ll look
and try again!

 

[Jay 1987]

It's a good clear diagram which I find pretty easy to read.
But it may not work exactly as you are expecting it to.

If you were to turn on all three lights by their switches, and then turn off light 2 only by its switch, what will happen?
It may help to sketch what happens on the diagram with a pencil,

It's a good clear diagram which I find pretty easy to read.
But it may not work exactly as you are expecting it to.

If you were to turn on all three lights by their switches, and then turn off light 2 only by its switch, what will happen?
It may help to sketch what happens on the diagram with a pencil,

Would I turn off the high powered mode also ?

 

[Lucien Nunes]

OK, I just looked on a bigger screen. Problem 2 is not as I stated. I thought the contactors were switching the fan neutral, which would have stopped it running at low speed with all the lights off. You have actually drawn a link from the supply N to the fan neutral commoning terminal, so I was mistaken about that. But, then, what is the wiring to terminals 3 & 4 supposed to achieve?

 

[Jay 1987]

OK, I just looked on a bigger screen. Problem 2 is not as I stated. I thought the contactors were switching the fan neutral, which would have stopped it running at low speed with all the lights off. You have actually drawn a link from the supply N to the fan neutral commoning terminal, so I was mistaken about that. But, then, what is the wiring to terminals 3 & 4 supposed to achieve?

Yes I found that problem out I have the supply Line going th

OK, I just looked on a bigger screen. Problem 2 is not as I stated. I thought the contactors were switching the fan neutral, which would have stopped it running at low speed with all the lights off. You have actually drawn a link from the supply N to the fan neutral commoning terminal, so I was mistaken about that. But, then, what is the wiring to terminals 3 & 4 supposed to achieve?

so now your saying I have a problem with supply for terminal 1 also what is the point of terminal 3 and 4 ?

 

[Jay 1987]

It's a good clear diagram which I find pretty easy to read.
But it may not work exactly as you are expecting it to.

If you were to turn on all three lights by their switches, and then turn off light 2 only by its switch, what will happen?
It may help to sketch what happens on the diagram with a pencil,

I don’t know mate from what I can see high powered mode would still work even if I turned off switch number 2. The only thing I can see as a potential Problem is terminal 2 may still be energised. But I don’t see how that affects anything when the light switch is off the circuit is broken. that means the coil should de-energise.

 

[Lucien Nunes]

Yes. You don't want to switch the neutral as the fan runs all the time, so you don't need the second pole (3 & 4) of the contactors. At the moment you have wired those contacts up, but then short-circuited them with a link from the supply neutral to the N commoning terminal. So those connections are completely redundant. The coils' A2 terminals and the fan need a neutral, and that's it.

But the problem Davesparks highlighted is what is commonly called 'backfeeding'. Suppose switches 1 & 2 are turned on, energising both of their contactors. Each will send a feed from the light switch through terminals 1 and 2 into the fan SL and the fan will speed up. So far so good. But when switch 1 is turned off, its light and contactor coil are still connected to the fan SL, which is still live as switch 2 and contactor 2 are still energised. What happens?

E2A having seen your last post. Yes, terminal 2 will still be energised from the other switch(es) that are on. Now consider that current will flow either way through the contactor contacts...

 

[Jay 1987]

Yes. You don't want to switch the neutral as the fan runs all the time, so you don't need the second pole (3 & 4) of the contactors. At the moment you have wired those contacts up, but then short-circuited them with a link from the supply neutral to the N commoning terminal. So those connections are completely redundant. The coils' A2 terminals and the fan need a neutral, and that's it.

But the problem Davesparks highlighted is what is commonly called 'backfeeding'. Suppose switches 1 & 2 are turned on, energising both of their contactors. Each will send a feed from the light switch through terminals 1 and 2 into the fan SL and the fan will speed up. So far so good. But when switch 1 is turned off, its light and contactor coil are still connected to the fan SL, which is still live as switch 2 and contactor 2 are still energised. What happens?

E2A having seen your last post. Yes, terminal 2 will still be energised from the other switch(es) that are on. Now consider that current will flow either way through the contactor contacts...

Mate, I’ve had enough today I’ll get back to you tomorrow

 

[davesparks]

I don’t know mate from what I can see high powered mode would still work even if I turned off switch number 2. The only thing I can see as a potential Problem is terminal 2 may still be energised. But I don’t see how that affects anything when the light switch is off the circuit is broken. that means the coil should de-energise.

The coil should de energise, but it won't.
The switch will be off (open) but the relay will remain energised because it will backfeed from the other switched lives.
The relay will be latched in that position, and the light will also stay on, until all three light switches are turned off.

Having terminal 1 directly connected to the coil defeats the purpose of the relay being in the circuit.

 

[Jay 1987]

Here’s my third attempt, taking into consideration all the feedback!

 

[Jay 1987]

Here’s my third attempt, taking into consideration all the feedback!

Is this attempt right? Or is it still wrong ?

 

[Lucien Nunes]

Spot on. Each light controls only the coil on its contactor. All three contactors have their contacts in parallel so that any one of them energised will operate the fan.

Of course, the neutrals of the contactor coils should really go back to the lighting circuit because neither that nor the fan circuit can be correctly isolated because of the interconnection by the 'borrowed' neutral. But that is how it was installed as we considered above.

One final point about the drawings. Consider the difference between
a) a wiring diagram and
b) a circuit diagram or schematic.

The wiring diagram shows each physical wire and where it connects to. This is good for showing how something is assembled and is used in production environments. Complex diagrams can be cluttered and difficult to follow. Put into words: N to Con 1 A2. Con 1 A2 to Con 2 A2. Con2 A2 to Con 3 A2. Etc.

The circuit diagram shows symbolically how things are connected together, in the clearest possible way, without specifying where each wire runs. This is good for engineering and troubleshooting purposes. In words: Con 1, 2 & 3 terminals A2 are all connected to N.

Get a working knowledge of both conventions...

 

[Jay 1987]

Spot on. Each light controls only the coil on its contactor. All three contactors have their contacts in parallel so that any one of them energised will operate the fan.

Of course, the neutrals of the contactor coils should really go back to the lighting circuit because neither that nor the fan circuit can be correctly isolated because of the interconnection by the 'borrowed' neutral. But that is how it was installed as we considered above.

One final point about the drawings. Consider the difference between
a) a wiring diagram and
b) a circuit diagram or schematic.

The wiring diagram shows each physical wire and where it connects to. This is good for showing how something is assembled and is used in production environments. Complex diagrams can be cluttered and difficult to follow. Put into words: N to Con 1 A2. Con 1 A2 to Con 2 A2. Con2 A2 to Con 3 A2. Etc.

The circuit diagram shows symbolically how things are connected together, in the clearest possible way, without specifying where each wire runs. This is good for engineering and troubleshooting purposes. In words: Con 1, 2 & 3 terminals A2 are all connected to N.

Get a working knowledge of both conventions...

Thanks lucien, it was a bit of a slog, but got there in the end!

I believe understanding diagrams really opens up your electrical design abilities,which in turn makes you a better spark. This is something I really enjoy.

it would be great if more of these tasks could be set. I have no commercial 3phase experience yet, but it would be good to know how installations work in that type of environment.

maybe yourself or another mentor could put some scenarios together that would require circuit drawings or calculations for commercial.

that would be great

Never the less thanks for the support.

jay