Help with earthing circuit

[Leccy monster]

Hi everyone

I'm a DIYer with little electrical knowledge looking for some advice I'm currently building a sterilisation cabinet with a UVC light in it. The cabinet is a Wooden box with a hinged lid I plan to use a microswitch to cut the uv lamp when the lid is opened.

Am I correct to think I can have a power lead coming in to a connecter block, take the + wire to the microswitch and back to the connector block and then take all wires + - and earth from connector block to the ballast unit? With the earth then connect to the ballast? Or would I need some special arrangement to earth it? The ballast unit will be mounted on the box and the earth will connect to an earth port on the ballast unit would this be sufficient to ground the circuit? See sketch.

All help much appreciated!

 

[HexHex]

You are taking the in and out of the switch to the same connector so the switch will do nothing

 

[Leccy monster]

You are taking the in and out of the switch to the same connector so the switch will do nothing

Ah I see what you mean sorry the drawing is bad it should look like this. Would this work?

 

[ipf]

Think about the switch and what it should do.

That rough drawing looks a bit like a machine gun......upside down

 

[Leccy monster]

Think about the switch and what it should do.

That rough drawing looks a bit like a machine gun......upside down

Doesn't the switch just need to break the circuit? Does it need to break positive and negative? The 1st sketch was wrong it should look like this one so that it completely breaks the positive connection

 

[Lucien Nunes]

The mains is AC, there is no positive or negative. There are line and neutral, a single-pole switch breaks only the line. If the interlock microswitch is a safety device, your risk assessment should indicate whether a single or double-pole interlock is needed, and whether the switch needs special characteristics such as guided contacts to ensure the requirements for safety are met. If not, and it is merely functional switching, a single-pole switch is fine and preferable.

 

[pc1966]

Generally it looks OK, but it is a bit odd having the "safety" switch as the main functional switching part. In this case though I guess you are really just making sure no one gets UVC in their eyes.

How do you time it for sufficient sterilisation? It might make sense to look at adding some sort of timer as well so you can set 5min (or whatever) and forget about it.

You mentioned it is made of wood so other than the ballast I presume there is no metal with any risk of getting close to a live wire to be earthed?

Finally make sure all the terminals are properly boxed / shrouded so there is not possible risk of touching them, and also think if there is any water or other liquid involved that would present a risk of getting in to the switch.

Wago make plastic boxes and various matching terminal blocks for joining stuff you might want to look at using, but if it is inside a box without risk of contact then standard screw 'terminal strip' joints can be used. Just don't forget to secure the main cable mechanically to the overall box before the outer sheath is removed and it reaches the terminals.

 

[Leccy monster]

Generally it looks OK, but it is a bit odd having the "safety" switch as the main functional switching part. In this case though I guess you are really just making sure no one gets UVC in their eyes.

How do you time it for sufficient sterilisation? It might make sense to look at adding some sort of timer as well so you can set 5min (or whatever) and forget about it.

You mentioned it is made of wood so other than the ballast I presume there is no metal with any risk of getting close to a live wire to be earthed?

Finally make sure all the terminals are properly boxed / shrouded so there is not possible risk of touching them, and also think if there is any water or other liquid involved that would present a risk of getting in to the switch.

Wago make plastic boxes and various matching terminal blocks for joining stuff you might want to look at using, but if it is inside a box without risk of contact then standard screw 'terminal strip' joints can be used. Just don't forget to secure the main cable mechanically to the overall box before the outer sheath is removed and it reaches the terminals.

Thanks for your feedback my Initial plan was to just switch it on and off at the mains and the microswitch just to prevent accidental exposure but Timer is a great idea.

The only metal are some brackets on the exterior. Should I earth these? I plan to use terminal blocks to house the connections so they aren't exposed the ones I'd seen were from screwfix.

 

[pc1966]

You only need to earth the external metal if it is possible it could become live under fault conditions. Realistically if it is a wooden box, your wires are all several cm away, adequately insulated themselves, and clipped in place, that would be 'no'.

 

[UKMeterman]

UVC is nasty, you do need two seperate safety switches to cover for a number of safe under single fault conditions. Can't you just buy a unit ready built?

 

[freddo]

Here's my UV-C lamp, for getting suntanned sunburns at home.

 

[pc1966]

Here's my UV-C lamp, for getting suntanned sunburns at home.

Wow - that is some, er, interesting kit.
And the £15 price tag at that time was serious cash!

 

[freddo]

It's quite severe, soon get that burnt pork smell when skin is exposed, even for a few seconds. It also spews out what I can only assume are hazardous amounts of Ozone. At least it comes with the original safety goggles... Most vintage medical stuff just seems to make your health worse.

 

[telectrix]

yrears ago i remember we had a tanning lamp, 1 element for warmth, and the UV was generated by a carbon arc. you screwed the electrodes together till they shorted, then screwed them back a bit to get a good sizzling arc. doubt it could get a CE mark nowadays. it was bloody lethal.

 

[Leccy monster]

OK thanks for the advice. I now plan to use 2 microswitch for the lid for extra security and a timer switch so the unit automatically switch off after certain elapsed time say 10 min. One more question does only the live wire feed to the switch or does the switches need live and earth? I want to use single pole micro switch.

This is my new sketch diagram

 

[pc1966]

OK thanks for the advice. I now plan to use 2 microswitch for the lid for extra security and a timer switch so the unit automatically switch off after certain elapsed time say 10 min. One more question does only the live wire feed to the switch or does the switches need live and earth? I want to use single pole micro switch.

For functional switching it is perfectly normal to only switch the phase (live) conductor, but that is not providing isolation that would be needed for any work on the unit such as replacing the UV tube.

If you are building this just for yourself to use that is probably OK, but it is always a good idea to put a warning on if others might ever try to use it to the effect that you must switch off and unplug it before replacing the tube. The "others" who are unaware of the limitations of the switching might be you in 10 years time!

 

[freddo]

Looks ok, make sure all the terminals are insulated.

yrears ago i remember we had a tanning lamp, 1 element for warmth, and the UV was generated by a carbon arc. you screwed the electrodes together till they shorted, then screwed them back a bit to get a good sizzling arc. doubt it could get a CE mark nowad
ays. it was bloody lethal.

I have one of those, I did wonder if it ever had a guard, mine is missing. The element is mostly to act as a resistor as the arc is basically a short circuit. The adjustment knobs have exposed live parts, and the little metal plate at the bottom only stops some of the carbon sparks from going on the carpet. And it's best not to mention the asbestos the element is wound on.

View: https://youtu.be/GhmhX4Bne30

 

[telectrix]

those switches don't need earthing, but each cable needs an earth conductor and the earth must be continuouse. i.e. connect the in and out with a connector art the switch position/s.

 

[Leccy monster]

those switches don't need earthing, but each cable needs an earth conductor and the earth must be continuouse. i.e. connect the in and out with a connector art the switch position/s.

What do you mean earth conductor?
Thanks

 

[pc1966]

What do you mean earth conductor?
Thanks

The earth conductor from the 13A plug to the ballast, etc, for protection.

I think Telectrix means is it should not have any joints that could come loose, such as screw terminals, etc, as that presents an additional hazard. Of course a lot of equipment has the earth cable in to a screw terminal when it enters the equipment, but that is another story...

Ways of joining that are considered permanent (i.e. making two wires in to an "continuous conductor") include soldering and crimping, assuming either is done competently. Some of the spring loaded joints (e.g. Wago) are considered "maintenance free" as they do not loosen with vibration or thermal cycling, so that is another way.

But the original (and some say best) is to use a wire where you strip a bit of insulation mid-way, and bend that in to a protruding conductor that can then be screwed in, etc. That way is the screw comes loose you disconnect from the terminal (as expected) but the downstream joints are still properly earthed. (Can't find a picture just now to show this, maybe someone else can?)