Protective Devices

[Marvo]

There's hundreds and hundreds of different protective devices around that you're likely to come across sooner or later. They all operate slightly differently and they may appear different in shape and size but they've all got one thing in common in that they will facilitate the disconnection of the supply to a circuit if a dangerous situation occurs.

Some devices are direct disconnection devices ie the supply to the circuit actually runs through them and they have an internal contact to allow disconnection, some are indirect ie they send a signal or interrupt the supply to another device (maybe a contactor for example) that actually does the donkey work of disconnecting the circuit.

Disconnection devices could be automatic reset so they will automatically re-close the circuit after the dangerous condition has subsided or after a certain period of time, others are manual reset which requires input from a person to re-energise the circuit hopefully after testing and remedial work has been performed.

I've picked 10 disconnection devices pretty much at random from the internet and posted their photos below. What I'd like the trainees to do is name each one of them and give the following info about them;

1. What is the device generally called?
2. What does the device sense and how?
3. Does the device provide direct or indirect disconnection?
4. Is the device manual reset, auto reset, neither or could be both?
5. If the device has disconnected does it qualify as safe isolation of the supply?
6. In what application (where) are you likely to come across one of these devices?
7. What about this device makes it so suitable for its application?
8. Give at least one interesting piece of random info about this device.

Okay, here's a quick example;



1. What is the device generally called?
It's known as an RCD which is something of a blanket term.

2. What does the device sense and how? What does it ultimately protect against ?
It senses the current flow through the live and the neutral side of the circuit by means of a small CT and some tricky electronics. Any imbalance detected indicates an earth leakage current. The device will trip should the earth leakage current exceed a certain threshold. It ultimately protects people from being killed by electrocution.

3. Does the device provide direct or indirect disconnection?
Direct disconnection on both L+N sides of the circuit (2-pole).

4. Is the device manual reset, auto reset, neither or could be both?
Could be both. This particular device shown has an auto re-closer which will reset automatically for up to 3 attempts.

5. If the device has disconnected does it qualify as safe isolation of the supply?
If it tripped due to a fault then 'NO' it does not provide safe isolation because it could auto re-close at any time. If it's safely operated and locked out then 'YES' it does qualify as safe isolation.

6. In what application (where) are you likely to come across one of these devices?
Protection of final circuits. Possibly domestic but pretty unlikely. More likely in a commercial installation where power fail is likely to cause stock losses.

7. What about this device makes it so suitable for its application? What about the device might make it unsuitable?
It's accuracy and it's speed of disconnection makes it effective at it's job.

8. Give at least one interesting piece of random info about this device.
RCD's were originally pioneered in South Africa with the intention of reducing the high number of deaths of people working on the mines.

Just out of interest when was that last time you checked an RCD to make sure it doesn't have an auto-recloser when you were using it to isolate a circuit? Maybe a discussion for another day...

This isn't a competition so feel free to to give info on some of the devices if you're unsure about others and even if info has already been given about a device you can always add more. My advice is to jump straight in and claim a couple of the easy ones for yourself, if you procrastinate for too long there will only be tricky ones left.

Here's the 10 devices in no particular order, you can quote this post and add your answers in bold or italics.

#! Done by Pennychew

#2

#3 Done by Marvo

#4

#5

#6 Done by Jamie

#7 Done by Happysteve

#8

#9

#10

 

[Jamie]

#6

1. What is the device generally called?
Thermal Overload.

2. What does the device sense and how?
Senses a rise in temperature through the heating elements internally, proportionally to the current being drawn by a motor.

3. Does the device provide direct or indirect disconnection?
Direct disconnection of all three switch mechanisms (attached to heating elements), and in turn disconnection of all three phases.

4. Is the device manual reset, auto reset, neither or could be both?
(Device specific) Manual reset

5. If the device has disconnected does it qualify as safe isolation of the supply?
No, it does not qualify as safe isolation of the supply THEY ARE EASY TO BYPASS

6. In what application (where) are you likely to come across one of these devices?
Motor control circuits - Industrial Applications

7. What about this device makes it so suitable for its application?
Designed to attach directly onto contractors, which control motors or high load industrial lighting circuits

8. Give at least one interesting piece of random info about this device.
Symbol for a thermal overload:
http://i.Upload the image directly to the thread.com/a7NxBD4l.png

Symbol integrated into a schematic:
http://i.Upload the image directly to the thread.com/0p58RFul.png

 

[Marvo]

Superb post Jamie. Great info, The symbols and schematics are a nice flashy touch.

The only thing I might add because it's safety related is that most thermal overloads have a selector switch for manual or auto reset mode and this would be an additional reason they're not suitable to consider as safe isolation.

 

[Marvo]

Okay girls and boys. Roll up roll up, only 9 protective devices left. Grab one while you can.

 

[happysteve]

#7

1. What is the device generally called?
Thermal fuse, or thermal cut-out.

2. What does the device sense and how?
Senses ambient temperature. Under normal (low temperature) conditions, a non-conductive pellet inside the device keeps a spring in a position where it shorts the two leads together. Once the device reaches a certain temperature, the pellet melts, allowing the spring to relax, leading to an open circuit.

3. Does the device provide direct or indirect disconnection?
It is a one pole device. It provides direct disconnection.

4. Is the device manual reset, auto reset, neither or could be both?
From the look of it, I don't think you can reset this device (one use only, like a regular fuse). However, I can imagine that some devices will automatically reset once the temperature goes down again.

5. If the device has disconnected does it qualify as safe isolation of the supply?
It might do, but I wouldn't trust it. I'm gonna go with "no."

6. In what application (where) are you likely to come across one of these devices?
Inside an electrical appliance. Could provide backup protection for temperature controlling thermostats; fan heaters, refrigerators, industrial equipment (thermal safeguard).

7. What about this device makes it so suitable for its application?
(assuming it's the "one use only, melting pellet" variety) - it's a direct mechanical connection, resulting in very low "on" resistance and pretty close to infinite "off" resistance. And they're cheap (~10-50p each).

8. Give at least one interesting piece of random info about this device.
You need to be very, very careful when soldering these to your circuit board. Think about it...

 

[Marvo]

Spot on Happy Steve. I thought this might be one of the last devices for people to have a go at but you nailed it early. As you say it's just a thermal fuse. It doesn't reset and it only protects against high temperatures, not over-current. They're used in millions of household appliances such as steam irons and maybe microwave ovens although not so much in fridges and they're just the last line of safety against the thing bursting into flames. 9 times out of 10 if one of these goes open circuit there will be another underlying cause that made it overheat. They can usually be replaced but you need to find the reason it overheated in the first place otherwise you're on a hiding to nowhere. People have a nasty habit of just wiring them out and the appliance will then be a fire hazard.

Just for reference if one of these is blown open circuit there's no way you should consider it as safe isolation of the supply.

You're obviously on a roll so pick another one if you've got the time and the energy

 

[happysteve]

Thanks for that

To fess up: I had no idea what it was when I saw it! I just googled some of the characters on the device, found out what it was, then read up about it. Haven't come across these on my course, but I've certainly learnt summat today.

Jamie set a very high bar with the first answer!

 

[Marvo]

Yep, Jamie threw a blinding first reply in which certainly set a high standard.

TBH I really don't mind if you google them, as long as you learn something it's mission accomplished. I didn't expect, especially as trainees, that you would have come across a lot of them before.

 

[Marvo]

Okay, it's Saturday evening an I'm sure you're planning an alcohol fueled night of general debauchery and impregnation of females. Think how much better you'll feel about yourself if you've shown your prowess by taking one of the remaining protective devices well and truly off the table. It's a well known fact that women are more responsive to confident, self assured guys.....think about it.....what have you got to lose????




**edit**I particularly liked the photo above because it looks like the chick has got at least 3 legs if you look closely

 

[Marvo]

Okay, c'mon guys, there's still 8 devices left and they think they've got the better of you. I heard them talking amongst themselves and sniggering. I wouldn't take it laying down if I were a trainee.

The score at the moment is Trainees;2 Protective Devices;8. Time to show them who's who in the zoo me thinks!!!!

 

[pennychew]

Okay, c'mon guys, there's still 8 devices left and they think they've got the better of you. I heard them talking amongst themselves and sniggering. I wouldn't take it laying down if I were a trainee.

The score at the moment is Trainees;2 Protective Devices;8. Time to show them who's who in the zoo me thinks!!!!

Ive got to apologise Marvo, its a great thread and you've obviously put some effort into it, it deserves a better response really! i cant speak for others but im totally snowed under at the minute.

I'm going to get my pizza out the oven (12" Chicago town 'The Takeaway' stuffed crust cheese and tomato for those out there that are wondering) then im going to google one of the devices while i eat it

 

[Marvo]

Sounds good to me. I'm heading to the fridge myself after that run down of your pizza. I'll let you know what treasures lie within although I doubt I'll be able to match your stuffed crust pizza

 

[pennychew]

Sounds good to me. I'm heading to the fridge myself after that run down of your pizza. I'll let you know what treasures lie within although I doubt I'll be able to match your stuffed crust pizza

If you werent so far away i would have dropped you a slice off. It would have been some fromage in homage to this thread

 

[Marvo]

Peanut butter (chunky)and morello cherry jam on toast.

 

[pennychew]

Im going to go with number 1, i had to reverse google the image to find out what this was as i had no idea!

1. What is the device generally called?

Klixon phenolic overload protective device.
Sensata Technologies’s line of automatic and manual reset protectors phenolic protectors are intended for ½ hp to 15 hp general purpose, commercial and industrial motors. These single–phase and three–phase protectors can be used in single and dual voltage type motors.
2. What does the device sense and how?
Klixon phenolic motor protectors are equipped with a bimetallic snap–acting disc, on which the contacts are mounted, and through which the current flows. If overheating conditions occur, the heating effect of the current flow through the Klixon disc and the influence of motor heat will cause the disc temperature to rise.

When the disc reaches the calibrated setpoint, the Klixon protector automatically opens and shuts down the motor, limiting the winding and shell temperature.

When the motor has cooled to an acceptable operating level — allowing the protector to cool to its reset temperature — the Klixon protector resets automatically to a closed contact position. This allows the motor to restart.

3. Does the device provide direct or indirect disconnection?
I would say indirect as its used in the control circuit
4. Is the device manual reset, auto reset, neither or could be both?
It could be both
5. If the device has disconnected does it qualify as safe isolation of the supply?
No
6. In what application (where) are you likely to come across one of these devices?
Fridge compressors, air conditioning and heating
7. What about this device makes it so suitable for its application?

Sensitive to both temperature and current
•	Precision calibration
Protects ½ to 5 HP motors
Automatic or manual reset
Fractional to 10hp
Easy to install
UL recognized, CSA certified, VDE certificate

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8. Give at least one interesting piece of random info about this device.
trying to find out what it is, what it does and how it does it meant that half of my pizza had gone cold before i had eaten it.

More info can be found here... http://www.sensata.com/klixon/motor-protector-phenolics.htm#uses

 

[Marvo]

Nice!!! Pizza with a protective device for dessert...can't beat it

I'm impressed with the choices, this is another one I expected to be chosen only after the fuses and motor starter had gone.

These things can be found in just about every home, anyone with a domestic fridge or chest freezer has got them. They're specifically designed to be mounted hard against the compressor dome because as you said they not only measure current to protect against overload caused by failure to start (locked rotor) they also directly measure the temperature of the body of the compressor and protect against overheating due to blocked condenser or condenser fan failure or some other refrigeration problems.

Almost all of them are auto-reset and switch back on when they've cooled down. There are manual reset versions available of the same device but they look different to the one in the picture.

In domestic fridges and freezers these things actually switch the supply to the compressor so they're configured as direct-switching and their contacts are capable of disconnecting and reconnecting the full load current or even the locked-rotor current which could be 5 times higher than the normal load current.


Just for info, if you ever come across one of these that's gone faulty there's at least a 90% chance something else / another problem has caused it. They generally only fail if they've been cycling on and off frequently for a prolonged period, under normal circumstances these devices never disconnect. If you encounter a fridge where one of these devices is constantly opening and resetting closed every couple of minutes it generally indicates that the compressor is seized and needs replacing.


Nice work Pennychew that makes the score 7;3 and the remaining devices aren't sniggering any more

 

[Marvo]

Okay, I'm getting a bit bored so I'll do one of them whilst I'm waiting.:39::winkiss:

#3

1. What is the device generally called?
It would probably be called an automotive miniature circuit breaker. It's actually a newfandangled replacement for the old car fuses.

2. What does the device sense and how?
It senses current by thermal action.

3. Does the device provide direct or indirect disconnection?
It disconnects the full load current of the circuit so it's a direct disconnection device.

4. Is the device manual reset, auto reset, neither or could be both?
Could be either. There's two different types available, one type automatically recloses the circuit after it's cooled down, the other type stays open circuit until it's physically removed, then it recloses and it can be reinserted.

5. If the device has disconnected does it qualify as safe isolation of the supply?
No, it could reclose automatically but 'safe isolation' doesn't really apply the same to 12V DC car circuits.

6. In what application (where) are you likely to come across one of these devices?
Protecting 12v DC circuits in a car, it would be located in the car's fusebox.

7. What about this device makes it so suitable for its application?
It protects against overload but it's reusable after a fault has occurred so you don't need to carry spares or buy new ones as was necessary with the older type fuses.

8. Give at least one interesting piece of random info about this device.
It has an internal bi-metallic strip which facilitates the opening and closing of the contact.