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SAFETY!!! How can I earth a Sinotimer programmable timer (TM618) inside a metal box ?? Answered

Hi, I am in the process of constructing a retro console timer for my 10yo grandson (for his parents really). They only want the power to the console on a predetermined times. I found an idea on Instructables which is basically a programmable 7 day timer in a small (lockable) cash box.

Question: there is no earth tab on the input side of the timer and I want to earth the box because it will contain 240v!! Is it as simple as wiring the wall plug with three core cable and attaching the spare earth wire to the inside wall of the metal box? Please advice.

Thanks

Discussions

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Jack A Lopez

4 weeks ago

Do you want someone to draw for you a picture, or diagram, of what the wiring to and from your timer gizmo should look like?

I was looking for some description of your timer gizmo. I think that Sinotimer (r) TM618 comes in a few different versions, depending on what powers it (e.g. 6VDC, 12VDC,24VDC,36VDC, 48VDC, 110VAC,220VAC).

I am guessing you have the version with 220 VAC input. It might have the suffix "N-2".

The attached pictures, of what I am guessing is a timer just like yours, were borrowed from this page at Scamazon,

https://www.amazon.com/dp/B07KPHPBBZ/

In particular, I am curious to know if the back of your timer looks like the back in the picture I attached.

By the way, the reason "there is no earth tab on the input side" of this timer, is because this timer is in a plastic case. You know, there is not too much danger of the case, buttons, etc, becoming electrified, even if wires inside break or come loose.

Did you bother to read the explanation I posted previously, about how grounding (you call it "earthing") works? I am guessing not.

Also, did you notice the output tabs on this timer gizmo? It is just a switch, a single pole single throw (SPST), or at least that is what the drawing on the case suggests. So it could be used to switch a variety of different kinds of power, besides mains power.

sinotimer-TM618N2-front+bottom.jpgsinotimer-TM618N2-back-wiring.jpg
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TN1946Jack A Lopez

Reply 27 days ago

Hi Jack, my timer is exactly the same front and rear except mine says TM618sH-2, not N2. I have ordered solder tags off eBay which I intend to pop rivet to the steel box and then solder the third input wire (earth) onto it.

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Jack A LopezTN1946

Reply 26 days ago

I have attached a picture of a grounding connection found, in the wild, erm... inside a microwave oven. It is not one of mine. I stole the picture from here:

https://dengarden.com/home-improvement/Waht-is-The...

This shows a typical way to do this. A lug crimps onto the wire, and this lug is held against the metal case by a screw or bolt.

Often there is a lock washer, or serrated washer, too,
https://en.wikipedia.org/wiki/Washer_(hardware)#Sp...
as that will keep the screw, or bolt, from coming loose. Also the the sharp edges on the washer will bite into the metal a little bit, which helps too, since it improves the electrical conductance somewhat.

Your plan, "to pop rivet to the steel box and then solder the third input wire (earth) onto it," is OK too I guess.

Although, I think it might be easier to do the soldering first, because it is harder to solder things bolted to metal, because the metal will tend to sink away the heat from the soldering iron, and make it hard to solder without pouring absurd amounts of heat into the metal box behind the solder connection.

Also I drew a picture-diagram of what I am imagining the wiring in your box should look like, if this timer is wired the way I think it is wired.

I used the usual colors for mains wiring in my home country, black for (L)ine, white for (N)eutral, and green for (G)round. The colors used in your country, or the wire colors found inside a hacked apart extension cord or power cord, might be different. E.g. I have often seen {brown, blue, green} inside the power cord for a desktop computer power supply.

Also, regarding colors, in my diagram I used red for the (L)ine switched on and off by the switch.

grounding-connection-inside-microwave-oven.jpgsinotimer-TM618N2-wiring-for-switching-mains-power.png
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TN1946

27 days ago

Jack, this is the description of the timer I have...

Description:
TM618sH 7 Days Programmable Timer Switch Weekly Digital Countdown Relay Time Controller
Features:
1.598-inch Super Large HD Display Screen.
Accurate to seconds, 12/24 system switching, countdown function, cancel/restore function.
Scope of application: household appliances, billboards, street lamps, spray paint control, aquarium feeding, regular pumping, etc.

Specifications:
Material: plastic
Size: 60*60*30 mm/2.36*2.36*1.18"
Control Range: 8 Switches (Daily/Week)
Working time error: 1s/24h 25℃
Blackout memory: 60 days
Internal battery: 1.2V 40mA (rechargeable battery)
Control power: 16A 250VAC
Specification:
100-130VAC 50/60Hz
200-250VAC 50/60Hz

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TN1946

4 weeks ago

Hi Downunder, I intended to pop rivet a tab through the box wall and then solder the earth lead onto that tab. Thanks.

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Jack A Lopez

4 weeks ago

Yes. It is as simple as you describe; i.e. connecting the ground wire, from the set of three conductors, {ground, neutral, line} found inside a three conductor power cord, to the inside of the metal box.

Look inside any mains powered appliance with a three conductor power cord and a metal case (e.g. desktop computer, microwave oven, etc.) , and you will see it is wired the same way.

The reason it is done this way, is because, speaking generally, there is fear of the wiring inside the case breaking, or coming loose, and the line conductor touching the inside of the metal case, and thus bringing the whole case to the same potential as the line voltage. If that were to happen, the user could be shocked by touching the metal case.

You might wonder at the possibility of wires breaking loose inside a metal box. However, I did say I was speaking generally. The scenario becomes more believable for those appliances with a motor, or other moving parts inside, like for example, a mains powered electric drill, or kitchen mixer, with a metal case. For something like that, it is actually likely the user would be touching the metal case, at the moment the motor breaks, and wires inside come loose.

So, if the case is tied to the ground wire, the case should never get close to line potential. Instead, when the line wire gets connected to ground wire; i.e. a ground fault, a big current flows through the ground wire. Usually this big current will trip the circuit breaker and turn off power to that circuit. More importantly, the resistance offered by the ground wire is much smaller than those paths available for current to flow through the user, so the vast majority of the fault current flows through the ground wire, and not through the user touching the appliance.

By the way, I would not be surprised at the possibility of a 10 year-old figuring out a way to defeat your crude mechanism for power control, e.g. finding an extension cord, picking the lock, or something we have not thought of, especially if those retro console games are fun to play.

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Downunder35m

4 weeks ago

Assuming the power supply complies with normal standards then it will be galvanically isolated between mains input and low voltage output.
That means you only need to make sure it is mounted properly so there is no chance to make and contact with the metal housing.
However if you want to add an earth connection due to being a metal box and preventing hram then it gets quite complicated.
That is if there is no suitable connection for the earth wire anywhere on the box ;)
In this case you need to add a srew or similar and just connect the earth wire from the mains cable to it.
This connects the metal from the box to ground and if something would happen your safety switch in your fuse box will trip and safe your life :)