hi all, i have some equipment that needs about 42 amps to start up for about 1-2 seconds. After starting it runs at 32 amps. However, its on 6mm cable and i dont want to be sticking a 50 amp etc breaker on it so it manages to start as i think 6mm wont handle a 50 amp if a fault ? Is there say a 32-35 amp mcb that will allow 1-2 seconds at 42 amps while starting?
wylex slow trip mcb?
- Thread starter mattysupra
- Start date
Not really relevant to my answer, but do you know the difference between overload current and fault current?
Even a B type is guarenteed not to trip on the magnetic part at anything less than 3XIn
S
sparkyspike
Use the mcb to protect the cable and the load. They will both be able to handle the extra current for a brief amount of time. 3xIn means 3 time the nominal current. What does the manufacturer specify, if anything?
i have no plates ect to tell me the load. They way i know what it pulls is by using a clamp meter.
What do you mean by use the mcb to protect the cable? Its on a 40 mcb at moment and sometimes it trips and sometimes it wont! Can i go higher? I did not think i could go higher than the cable rating ( even tho it only pulls 32 amp while running) . Can i stick say a 50 in there?
What do you mean by use the mcb to protect the cable? Its on a 40 mcb at moment and sometimes it trips and sometimes it wont! Can i go higher? I did not think i could go higher than the cable rating ( even tho it only pulls 32 amp while running) . Can i stick say a 50 in there?
S
sparkyspike
What curve are you using? B, C or D?
No, you must not put a 50A MCB in.
If you were to draw 50A from this circuit for a long period of time, the MCB would not trip. The cable would melt, and could cause a fire.
That is why you have to use an MCB which is the correct size for the CSA of the cable - i.e. use the MCB to protect the cable.
You may be able to use a 'C' type MCB, but before you can do this, you have to ensure the Zs for the circuit is low enough by testing with an earth fault loop impedence tester.
If you were to draw 50A from this circuit for a long period of time, the MCB would not trip. The cable would melt, and could cause a fire.
That is why you have to use an MCB which is the correct size for the CSA of the cable - i.e. use the MCB to protect the cable.
You may be able to use a 'C' type MCB, but before you can do this, you have to ensure the Zs for the circuit is low enough by testing with an earth fault loop impedence tester.
ye this is what i thought rf. Like is said before about protecting the cable.
Whats this b/c stuff all about?
Basically i have 6mm cable twin+earth that will have 32 amps being pulled down it for lets say 10mins a week. ( 8 meter run) But i need 42 amps to start up for 1-2 seconds!
What breaker do i need?
Whats this b/c stuff all about?
Basically i have 6mm cable twin+earth that will have 32 amps being pulled down it for lets say 10mins a week. ( 8 meter run) But i need 42 amps to start up for 1-2 seconds!
What breaker do i need?
MCBs are available with different tripping characteristics.
They are available in types 'B', 'C' and 'D'
They all have the same characteristics for disconnecting overload currents. This is where you draw a load larger that the rating of the MCB for a period of time. It warms the thermal part of the MCB which causes it to trip.
They have different characteristics for dealing with fault currents. This is where a very large current flows in a circuit, normally due to a short circuit. This operates the magnetic part of the MCB and causes it to trip.
These are available with different sensitivities. It will take a larger fault to trip a type 'C' MCB than it will to trip a type 'B'.
This is useful for appliances with medium start up currents such as motors and banks of fluorescent lighting, but as the magnetic part of the MCB is less sensitive, you need to ensure the earth to the circuit is good enough.
If the earth has a resistance which is too high, then in the event of a fault to earth, the circuit will not disconnect quick enough, so it is very important to verify this before replacing the MCB.
You can also get type 'D' MCBs, but these are normally only used for specialist installations with very high starting currents such as large welding sets, x-ray machines and some theatrical dimmers.
Hope that makes sense. I've tried to keep it in laymans terms.
They are available in types 'B', 'C' and 'D'
They all have the same characteristics for disconnecting overload currents. This is where you draw a load larger that the rating of the MCB for a period of time. It warms the thermal part of the MCB which causes it to trip.
They have different characteristics for dealing with fault currents. This is where a very large current flows in a circuit, normally due to a short circuit. This operates the magnetic part of the MCB and causes it to trip.
These are available with different sensitivities. It will take a larger fault to trip a type 'C' MCB than it will to trip a type 'B'.
This is useful for appliances with medium start up currents such as motors and banks of fluorescent lighting, but as the magnetic part of the MCB is less sensitive, you need to ensure the earth to the circuit is good enough.
If the earth has a resistance which is too high, then in the event of a fault to earth, the circuit will not disconnect quick enough, so it is very important to verify this before replacing the MCB.
You can also get type 'D' MCBs, but these are normally only used for specialist installations with very high starting currents such as large welding sets, x-ray machines and some theatrical dimmers.
Hope that makes sense. I've tried to keep it in laymans terms.
ye makes sence thanks RF. I already knew that sort of! but was not sure and you have confirmed it for me.
Do i need someone to come test the earth for me or is it something i can do myself and give you details back on here of my findings? What happens if it fails the earth test? Would i need to run seperate earth? Surely the earth in the 6mm is fine tho? Or not
So what breaker would i need? a 40 c rated?
Do i need someone to come test the earth for me or is it something i can do myself and give you details back on here of my findings? What happens if it fails the earth test? Would i need to run seperate earth? Surely the earth in the 6mm is fine tho? Or not
So what breaker would i need? a 40 c rated?
S
sparkyspike
If you can measure your earth fault loop impedance, see that it is less than 1.15 ohms if using a B-type, 0.57 ohms if using a C-type, 0.29 for a D-type.
S
sparkyspike
You need an earth fault loop impedance tester, and the test needs to be done live. Otherwise it's quite a simple test to do.
how do i measure this? If it have to i will get a spark in to do it for me, but i have had no confidence with sparks around brum now. Think i have been through at least 4 in the last few months that really have no idea!
I also have had another 3 in the last week giving quotes to sort my mcb out and non of them seem to know how to get around this problem . A couple have said i need to change to 10mm and put a 50-60 mcb on it but as it only pulls 32 amps runing i cant see the point in pulling all the floors up to do it! When i ask about a slow trip/delayed mcb they dont know what im on about!
Also the wiring in the panel is 6mm, so if i was to upgrade to a 50 amp mcb and 10mm breaker, surley the wiring in the panel will not be protected ?
I also have had another 3 in the last week giving quotes to sort my mcb out and non of them seem to know how to get around this problem . A couple have said i need to change to 10mm and put a 50-60 mcb on it but as it only pulls 32 amps runing i cant see the point in pulling all the floors up to do it! When i ask about a slow trip/delayed mcb they dont know what im on about!
Also the wiring in the panel is 6mm, so if i was to upgrade to a 50 amp mcb and 10mm breaker, surley the wiring in the panel will not be protected ?
S
sparkyspike
Stick with the existing rating of breaker, just change it to a C-type, if your B-type is tripping. If your 6mm cable is not too long and your EFLI to the DB is low, then you should be ok, but you need to test nonetheless, that a C-type will trip in good time. Do you know how to measure R1 + R2?
T
toasty
What appliance are we talking about here? Some kind of motorised power tool (large) I assume?
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