MCB's

NoseyMike2010

How do you determine what value of MCB's (Miniture Circuit Breakers) are required on a circuit? What steps needs to be determined?

grousedogtom

You need to have an idea of the total load of the circuit to be protected in amps or watts, or if you can tell me is it sockets or lights or shower or what the circuit is supplying

2011

It works a bit like this:

design current (A) < protective device rating (A) < current carrying capacity of the cable.

There are other factors that may require a larger cable to be used such as lenght of run, high ambient temperature etc.

NoseyMike2010

Any of you guys have an example or a circuit diagram that needs to show what MCB would be required to protect the circuit?

Or some mathematical formula that I can follow to understand MCB's better.

Cheers

2011

Try this:

http://boards.ie/vbulletin/showpost.php?p=57702831&postcount=3

NoseyMike2010

What is happening in the circuit? It still doesn't answer how he has determined which size MCB he should use.

Or have I completely missed the point!!!!!!!!!!!! :o:o:o

M cebee

2011 wrote: »

It works a bit like this:

design current (A) < protective device rating (A) < current carrying capacity of the cable.

There are other factors that may require a larger cable to be used such as lenght of run, high ambient temperature etc.

ya simplest way of explaining it
VD ,correction factors and fault-loop impedance need to be considered when sizing cable

2011

What is happening in the circuit?

Current flows down the live, through the load and back up the neutral. That is unless the current is larger than the rating of the protective device (in your case the MCB).

It still doesn't answer how he has determined which size MCB he should use.

It does. Let's say your design current is 6A. The next size up MCB is a 10A. The smallest cable size that will take 10 A safley and indefinitley (I think it can take about 19 A).

Or have I completely missed the point!!!!!!!!!!!!

Perhaps

Buford T Justice

What are you trying to wire up for OP?

Stoner

NoseyMike2010 wrote: »

Or have I completely missed the point!!!!!!!!!!!! :o:o:o

in short, the circuit will require a certain amount of electrical current to supply its load, or power requirement (Power = voltage x current)

You then select the cable size that can carry that current over that distance from the fuse/MCB board. (using tables in the latest wiring regulations)

You then select and MCB to protect that cable, i.e if the cable can carry 11Amps max you could select a 10Amp MCB to make sure that the cable does not burn out due to over current.

So if you want to find the cable size, and then the MCB size you need to know what you want to supply on the circuit and find out how much power it will use.

there are other consideration, regulations etc but this is the basic first step approach.

M cebee

Stoner wrote: »

in short, the circuit will require a certain amount of electrical current to supply its load, or power requirement (Power = voltage x current)

You then select the cable size that can carry that current over that distance from the fuse/MCB board. (using tables in the latest wiring regulations)

You then select and MCB to protect that cable, i.e if the cable can carry 11Amps max you could select a 10Amp MCB to make sure that the cable does not burn out due to over current.

So if you want to find the cable size, and then the MCB size you need to know what you want to supply on the circuit and find out how much power it will use.

there are other consideration, regulations etc but this is the basic first step approach.

load<mcb<cable

Randyleprechaun

MCB's are put in circuits to protect the cabling and the cabling only,

Therefore, the only thing you need to consider when trying to size a breaker is the cable size!!!! Discrimination of course should be taken into account but it is cable size that is of most importance.

Obviously, the size of the cable will be in line with loads, volt drops etc etc.

M cebee

Randyleprechaun wrote: »

MCB's are put in circuits to protect the cabling and the cabling only,

Therefore, the only thing you need to consider when trying to size a breaker is the cable size!!!! Discrimination of course should be taken into account but it is cable size that is of most importance.

Obviously, the size of the cable will be in line with loads, volt drops etc etc.

you consider the load when sizing the breaker

you consider the breaker when sizing the cable


if you choose the cable before the OCPD you may fiind the breaker is too small for the connected load with correction factors/loop impedance etc.

Randyleprechaun

M cebee wrote: »

you consider the load when sizing the breaker


you consider the breaker when sizing the cable

Cant agree:

You consider the load when sizing the cable, and then,

You consider the cable size when sizing the breaker.

M cebee

have a read of 1 thru 7

http://www.tlc-direct.co.uk/Book/4.3.9.htm

Randyleprechaun

I suppose it depends on what way you look at it.

The cable should be sized to counteract voltage drop and to be able to carry the load taking all factors into account.

After that, the MCB too will have to be sized to suit(as you have said MCeebee) your fault loop impedance and of course cable size.

2011

you consider the load when sizing the breaker

you consider the breaker when sizing the cable

+1

This is the correct order to do things.

NoseyMike2010

What are fuses needed for? And how do you determine what value fuse you need to protect a system?

I will scan some of my work later and hopefully it will show me grasping the idea of whats happening in the system!

Thanks to all for helping and replying :)

Buford T Justice

NoseyMike2010 wrote: »

What are fuses needed for? And how do you determine what value fuse you need to protect a system?

I will scan some of my work later and hopefully it will show me grasping the idea of whats happening in the system!

Thanks to all for helping and replying :)

Fuses are needed to protect the consumer from an electrical shock if there is a fault with an appliance or cable connected to the circuit.

2011

What are fuses needed for? And how do you determine what value fuse you need to protect a system?

let's say you have a circuit that under normal operation it should draw a maximum of 8 amps. To protect the circuit you would then install a 10 amp fuse. If all is well the fuse will not blow.

Imagine you had no fuse or protective device. If the circuit were to develop a fault and as result draw 30 amps you may have an issue because the load is melting but the selected can only safley rake 15 amps!

But if you had a fuse or MCB it would simply cut the power.

Think of fuses as being like insurance, you only need them if something goes wrong. When nothing goes wrong they may as well not be there.

Fuses can protect against overload and short circuit conditions. If protection against electric shock is your main concern start thinking about RCDs. Remember 0.05 amps at mains voltage has the potential to kill. Most domestic fuses are rated far above this. It is not normal to select fuses alone to protect against electric shock.

NoseyMike2010

So looking at the example I have attached. I have tried to put together this example with the help of the internet.

If you have a Fibre Optic module connected through to Terminals (I believe they are Weidmuller KDKS 1/32) and has an AI module (not sure at the top of my head what it is but I can check site if datasheet is needed) connected to it .

1) What exactly is happening in this circuit?
2) Why do you need to pick Weidmuller KDKS 1/32 terminals. What is their purpose and does the resistor need to be soldered in. Looking at the Weidmuller site tthere seems to be nothing connected in them!!! If so how do you calculate the value.

Hopefully someone can understand what I have done (or at least trying to do )

2011

Nosey:

what are you trying to build?

I don't know what the resistors are for. Perhaps they are not even resistors, I just don't know.

I think you are trying to run before you can walk.

First decide what it is you want to design and then someone here might be able to help you.

To understand the circuit shown first you must understand the difference between analogue and digital I/O.

Normally the PLCs that I work with have DIN rail mounted terminals with no electronic components in them at all. The exception is when we use what are known as I.S. barriers, but that is a whole other story.

Philistine

Hi NoseyMike2010,

Is the diagram part of a bigger system or something you put together yourself as a learning exercise (going by your other recent posts)?

I don't understand what the fibre optic part of the circuit does but the wiring is incorrect.
Firstly, I would suggest that there is no resistor in the terminals but maybe they are fused terminals?
Secondly, you don't have a complete citcuit as there should be two wires going from the "device" into the analog input (AI). Analog signals are usually 4-20mA, 0-20mA or 0-10v. You will damage the AI if you connect 24vdc to it.

NoseyMike2010

2011,

Good point ........

1) What are the difference between AI Modules and DI Modules. And how does a FOM (I guess thats Fibre Optic Modum) in general work. How does one talk to the other in this case?

2) I believe that this circuit (included in a previous post) that I drew reads the actions of the FOM. But I am honestly not 100% sure whats happening.

3) Looking at that circuit again ... maybe those resistors should be fuses. If they are fuses I would guess that they are there to protect the FOM. I am unsure how you would go about calculating their values?

4) I only added this miniture circuit to help me get started. I am trying to read up on the hardware design aspect and power electronics.

Cheers

Noseymike2010

NoseyMike2010

Philistine,

Its something I have pieced together as a learning exercise for myself. I have only tried to represent 1 AI channel being used with 1 terminal used and connected into the Fibre Optic Module.

I believe that the 24 V is connected to the fused terminals (and not resistors as you correctly pointed out). Is the idea of this fuse terminal to protect the AI module?

What would the two wires that go into each channel of the AI Module represent? I will examine my drawing again and try work this out.

Thanks for the help. This will get me thinking for tomorrow

NoseyMike2010

Attached is an updated drawing. I hope it is correct.

Philistine

To be honest, I don't think that circuit diagram makes any sense. Not on it's own anyhow. I'm open to correction but I don't believe an AI card can read fibre optic light pulses.

You've mentioned in other threads that you'd like to learn how to design, build and program PLC control panels.
Building these panels can't be learned by books. You really need to work with an electrician for practical hands on training for this. Programming a PLC is quite difficult and will take a long time to learn but again requires hands on training. Designing the PLC hardware architecture might be a good first step which doesn't require much practical training.

I'll get some example photos, electrical schematics etc... together over the weekend to get you started. But first, google and get yourself aquainted with P&ID's (process and instrumentation diagrams).

Basically, a mechanical or process engineer will design something and produce a P&ID which will not only show the mechanical elements of the project but also all the instrumentation required to operate it.
An electrical or controls or automation engineer will then "read" the P&ID to determine what PLC hardware is required. They will then produce electrical schematics.
An electrician will build the control panels as per the schematics and a software or controls engineer will write the PLC code.

This is a very basic description (leaving out many details and steps for now) so you have an understanding how a project is developed.

Don't worry about sizing cables and mcb's yet. Try to understand the PLC hardware is put together first. Check out manufacturers websites. Siemens, Allen Bradley, Mitsubishi, telemechanique, Omron etc...

NoseyMike2010

Philistine ... I would love to get to know the articuture first and learn PLC programming later on. I agree with you on getting practical experience. I had thought about doing the Electrician apprentship so I could get hands on experience.

I would like to know how to wire systems up and have a basic understanding of what is happening betweein each module.

Keep in touch .... thanks though for answering my questions and helping.

NoseyMike2010

Phil,

Did you manage to get those bits and pieces sorted? Cheers.

NoseyMike2010

2011

Basically, a mechanical or process engineer will design something and produce a P&ID which will not only show the mechanical elements of the project but also all the instrumentation required to operate it.

Well said Phil.

I am in the middle of this right now. It works like this:
1) From the P&IDs generate an instrument index in excel or access. Each instrument will have a tag no.

2) On the index have a field for all instruments with I/O

3) Filter out all instruments with no I/O such as gauges, relief valves etc. From this generate an I/O list.

4) Insert I/O type as which would be: Digital in, digital out, analogue in, analogue out, profibus etc.

5) From the I/O list work out the number and type of I/O cards required and from that the number of racks. 25% spare capacity is normal.

6) From the I/O list work out a cable schedule. Also look at layout and GA drawings to see where is best to locate Junction Boxes. This will save on cabling and cable containment.

Hope this helps!

2011

To be honest, I don't think that circuit diagram makes any sense. Not on it's own anyhow. I'm open to correction but I don't believe an AI card can read fibre optic light pulses.

I would agree. But there are modules that can convert a signal on a fibre optic cable to a 4-20mA signal and I suspect that that is what is shown in your drawing. Changing a signal from one form to another is quite common in instrumentation. It us known as "signal conditioning".

Philistine

NoseyMike2010 wrote: »

Phil,

Did you manage to get those bits and pieces sorted? Cheers.

NoseyMike2010

Hi, I'm working abroad at the moment so I don't have as much free time as usual. I'll get the info to you soon

Bruthal

Randyleprechaun wrote: »

Cant agree:

You consider the load when sizing the cable, and then,

You consider the cable size when sizing the breaker.

Id have to say i would consider the load when sizing the breaker, then the breaker when sizing the cable.

If a load is 35 amps then surely you would say ok a 40 amp breaker, and the next size cable now capable of continously carrying > 40A.

Because if you size a cable now to carry 35 amps then maybe you could pick a cable that can carry 38 amps,,, now what size breaker will you use?

Bruthal

2011 wrote: »

let's say you have a circuit that under normal operation it should draw a maximum of 8 amps. To protect the circuit you would then install a 10 amp fuse. If all is well the fuse will not blow.

Imagine you had no fuse or protective device. If the circuit were to develop a fault and as result draw 30 amps you may have an issue because the load is melting but the selected can only safley rake 15 amps!

But if you had a fuse or MCB it would simply cut the power.

Think of fuses as being like insurance, you only need them if something goes wrong. When nothing goes wrong they may as well not be there.

Fuses can protect against overload and short circuit conditions. If protection against electric shock is your main concern start thinking about RCDs. Remember 0.05 amps at mains voltage has the potential to kill. Most domestic fuses are rated far above this. It is not normal to select fuses alone to protect against electric shock.

It could also be said fuses offer better high current short circuit protection/interruption than MCB`s, which is why fuses are still used in MCB boards etc.

M cebee

robbie7730 wrote: »

It could also be said fuses offer better high current short circuit protection/interruption than MCB`s, which is why fuses are still used in MCB boards etc.

'traditionally' yes but i notice a lot of standard mcb's are gone up to 10kA now

Bruthal

M cebee wrote: »

'traditionally' yes but i notice a lot of standard mcb's are gone up to 10kA now

Are they, i never noticed that myself. I wonder how they manage that. Does something move between the contacts.

NoseyMike2010

Can u guys draw up a circuit diagram to explain what you are talking about please.

2011

'traditionally' yes but i notice a lot of standard mcb's are gone up to 10kA now

Yes but HRC fuses are about 80kA