How does a D curve breaker work

[Glovebox]

Hi all

I have a bench saw which is tripping a 100amp breaker on a workshop spread.

The question i have is how does a D curve breaker work? I know it has a longer trip time to deal with the start up of a motor. On this occasion a single phase saw with 14amp running load. Does the breaker stop the current passing through the coil so another breaker further up the line (the 100amp main switch) never sees its load until it drops back to normal running load of around 14amps or does the breaker let some of the current through meaning i exceed my 100amps and it trips

The problem i have is not all the time but occasionaly this will trip the main hundred amp breaker. This breaker runs many sub boards to include other tooling, air con units, lighting, a number of computers, UPS, compressors etc. It never trips the other two breakers of the sub boards its passes through before hitting the 100amp mains breaker.

Now these are not all on at once but i am wondering if this is purely a demand issue and i need to up the size of the 100amp breaker. possibly an extra air con is running and when the saw is turned it on it trips.

 

[James]

Some pictures of the breakers in question would be helpful
i would not expect a saw like that to trip a D type breaker if it is at least 16A breaker.
when it trips, is it almost instant when you press the start button or after a couple of seconds?

 

[Glovebox]

Some pictures of the breakers in question would be helpful
i would not expect a saw like that to trip a D type breaker if it is at least 16A breaker.
when it trips, is it almost instant when you press the start button or after a couple of seconds?

It doesn't trip the 16 amp d but the 100 amp c main breaker.

It has to be a loading issue. I just wonder if the d curve allows some current to pass through it so the 100amp is seeing just enough over current to trip?

 

[DPG]

Is thr main 'trip' an RCD? Could this be an earth leakage fault rather than overload?

 

[pc1966]

As @DPG suggested it might be a RCD style MCCB breaker, a photo might help.

If the saw is pushing the peak load to trip the 100A C-curve it sounds like it is already running over the rated 100A which is not good.

Under fault conditions with MCBs then you can see a 16A D-curve take out the supplying 100A MCB if the fault current is high enough, typically in the 1-2kA or above region, but then both breakers would go. With MCCB they often have a bit more "intetia" or short-term delay, so they are selective to much higher fault currents than a MCB (in some cases total selectivity, i.e. at any rated fault current the down-stream breaker goes alone).

 

[Lucien Nunes]

I know it has a longer trip time to deal with the start up of a motor.

It doesn't have a longer tripping time, the time response is the same regardless of whether it is B, C or D type. The difference between the different MCB types / curves is in the instantaneous trip current threshold. This is the current at which the magnetic tripping mechanism (designed to respond rapidly to high fault currents) takes over from the thermal tripping mechanism (designed to respond to long low overloads.) This is expressed as a multiple of the rated current. B-type trips instantly at 3-5x In, C-type at 5-10x In, D-type at 10-20x In. Until this threshold is reached, B, C and D type behave the same way: Must not trip at 1.13x In, Must trip within one hour at 1.45x In, Must trip within 1-2 mins at 2.55x In.

If the instantaneous trip of a D100 is being triggered the current must be at least 1000A which seems very unlikely! So either the breaker is faulty or it is tripping thermally due to excessively high sustained load, of which the compressor is just the final straw.

Does the breaker stop the current passing through the coil

MCBs have no influence over the current passing through until the moment they trip and open the circuit.

 

[Glovebox]

Is thr main 'trip' an RCD? Could this be an earth leakage fault rather than overload?

Just mcb

It doesn't have a longer tripping time, the time response is the same regardless of whether it is B, C or D type. The difference between the different MCB types / curves is in the instantaneous trip current threshold. This is the current at which the magnetic tripping mechanism (designed to respond rapidly to high fault currents) takes over from the thermal tripping mechanism (designed to respond to long low overloads.) This is expressed as a multiple of the rated current. B-type trips instantly at 3-5x In, C-type at 5-10x In, D-type at 10-20x In. Until this threshold is reached, B, C and D type behave the same way: Must not trip at 1.13x In, Must trip within one hour at 1.45x In, Must trip within 1-2 mins at 2.55x In.

If the instantaneous trip of a D100 is being triggered the current must be at least 1000A which seems very unlikely! So either the breaker is faulty or it is tripping thermally due to excessively high sustained load, of which the compressor is just the final straw.



MCBs have no influence over the current passing through until the moment they trip and open the circuit.

On you're last paragraph the D curve MCB must influence the current as surely that's what other there to do. Resist that current to let the motor start without tripping the main fuse at 100amp

There are no rcds involved in this problem.

The saw itself may also be knackered

 

[Lucien Nunes]

On you're last paragraph the D curve MCB must influence the current as surely that's what other there to do. Resist that current to let the motor start without tripping the main fuse at 100amp

No, it is only there to break the circuit when there is a fault or overload. The relationship between D-curve and motors is simply that the the D-curve has the very high instant-trip threshold so that it does not falsely detect inrush current as a fault. But the price to pay for that low sensitivity is that the circuit loop impedance has to be very low so that it trips when there really is a fault.

Anything to do with limiting the start current is a function of the motor starter.

 

[Julie.]

Hi all

I have a bench saw which is tripping a 100amp breaker on a workshop spread.

The question i have is how does a D curve breaker work? I know it has a longer trip time to deal with the start up of a motor. On this occasion a single phase saw with 14amp running load. Does the breaker stop the current passing through the coil so another breaker further up the line (the 100amp main switch) never sees its load until it drops back to normal running load of around 14amps or does the breaker let some of the current through meaning i exceed my 100amps and it trips

The problem i have is not all the time but occasionaly this will trip the main hundred amp breaker. This breaker runs many sub boards to include other tooling, air con units, lighting, a number of computers, UPS, compressors etc. It never trips the other two breakers of the sub boards its passes through before hitting the 100amp mains breaker.

Now these are not all on at once but i am wondering if this is purely a demand issue and i need to up the size of the 100amp breaker. possibly an extra air con is running and when the saw is turned it on it trips.

It looks to me that you have a capacity issue with your 100A mcb.

If this is already running at ~95-100% of capacity, and then the saw motor starts, it pushes it over.

There are three ways you can do this, one is a direct measurement of the current, you would need an electrician with a clamp meter (or to install current transformers and some monitoring equipment).

The second is to audit the loads throughout the system.
This is something that you can do, but is easy to miss something in the audit, you would need to see what is/was running and add up all the loads (so 1kW heater in office, 2kW air con motor etc etc) - however this actually takes some experience as a 1kW heater would add 4.3A , but a 1kW motor may add 6.5A - you need to understand load characteristics somewhat

The third method is a bit easier to do, but involves some maths, and relys on you having access to the main meter (and disconnecting anything else other than anything supplied via the 100A mcb), quite simply whilst everything is running as normal, time how long the meter takes to measure say 1kWh.

So 1kWh is 1kW in 60mins, - 60kWmins

If it took 3 mins to obtain 1kWhr, then the actual load must be 20kW
(1kW x 60 mins = 1kWh = 20kW x 3 mins)

Assuming single phase I = kW/voltage or 20,000/230 = 87A , this could very well be too much if the loads are motors as motors have a power factor (pf) usually around 0.7 - 0.8 - say 0.75, which means when you measure the power it's actually only 75% of what the current is likely to be - in this case around 116A - so too much for a 100A mcb, if the load is just heaters, then 87A is likely to be the actual current

(If its 3 phase, then the calculation is slightly different
I = kW/[voltage x 3] or 20,000/[230 x 3] = 29A)

The most accurate is direct measurement by an electrician with a clamp meter of course.

If the upstream mcb is tripping, then it is either faulty, or you have too much current.

 

[David Prosser]

It would be odd to find that only this saw is causing a loading issue in a workshop with, to quote the OP "This breaker runs many sub boards to include other tooling, air con units, lighting, a number of computers, UPS, compressors etc". I would of thought that a thermal tripping overload problem would manifest itself randomly when any number of different pieces of equipment are running.