- Joined
- 27 Jan 2008
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- 25,068
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- Location
- Llanfair Caereinion, Nr Welshpool
- Country
As you have said, there is no reason why one should not use total magnetic tripping set exactly on the threshold and use some damping device like oil and a plunger in a cylinder to delay the action in order to allow for the surge when first switched on. By selecting what viscosity of oil is used in the dash pot one could adjust the delay before tripping, I am 100% sure this would work, as I have actually seen it used.
However there is a reason why it is no longer used, and not solely cost, time has shown using a simple temperature controlled device in the main works better with less maintenance. I would think around 1985 was last time I topped up a dash pot, I seem to remember that transformer oil is cariogenic and items like motor starters and welding pods were always greasy with the oil on the outside, fire resistance fluid is still used, last time I saw a spill it caused the building to be closed down for a week for cleaning.
Over time I have found the simple fuse is still a good protective device, I have worked where machines were modified to reduce down time, one modification was to replace the contactor with a solid state device, however this produced two other problems, one was it could no longer be used for the E-Stop so we had to introduce an E-Stop contactor, and second we found from bitter experience when the heaters failed and went short circuit it would take out the solid state device every time in spite having MCB's fitted. The cure was to replace the MCB's with semi-conductor fuses, they would rupture faster than a MCB and would save the solid state device from failing.
I am not talking theory, it was shown in practice the fuse was a better protective device than the MCB. There is a problem mixing the MCB and the fuse, it is hard to get the discrimination required, back in 1980's simple bulbs had a fuse-able link built in so if when the bulb reached end of life there was ionisation in the bulb the link would rupture so the rest of the lights on the circuit would continue to operate as the link would rupture before the main fuse would blow. As fuses were replace in the distribution unit for MCB's it was found that the MCB was faster than the fuse link in the bulb, and when they failed with ionisation it would cause all lights on the circuit to fail rather than one single lamp.
However as time went on we stopped getting British made bulbs, and the imports often lacked the internal fuse, renewing the distribution box fuse became more regular so we had to go over to the MCB so it could be easy reset, it was nothing to do with MCB protecting better it was the problem with using inferior bulbs, and the cost of replacing the cartridge or uncertainty of the correct wire being used to replace the blown fuse wire which forced us to move over to the MCB. The MCB is often faster acting than the fuse, but it can also be slower acting, the MCB has a very defined point where thermal takes over from magnetic protection. The prospective short circuit current is extremely important, go under that critical value by just 1 amp and the time jumps from 0.01 seconds to 5 seconds. This is not a problem with the fuse. So as we went over to the MCB then we also had to carefully measure the PSCC or loop impedance just a little under or over the limit and the device failed.
Today we have many overload protective devices, including electronic used for motor protection built into inverters and soft starts, some only protect the device targeted, the inverter protects the motor, but not necessary the cable or personnel working around it. So we have to design, it is not a case of we always use a xyz, we have to consider what is required, we are then often required to sign paperwork to say how we have the skill to do that design and accept the responsibility for our actions. We are guided by the books which have charts to assist us, like BS7671, where it suggests the time which will be needed for the automatic disconnection of the supply should be within laid down limits.
We don't always get it right, I have seen many times specially with reduced low voltage where people have got it wrong, putting a 10A overload into the supply of a 240 to 55 - 0 - 55 volt transformer means the output will need to exceed 43.5 amp to earth before the input device will operate, yet the output is through a 16A socket. I would like to ban all yellow brick transformers they are dangerous devices and don't comply with regulations, at least until they start putting the over load device on the output. But it's not the fuse or overload at fault, it is the design of the yellow brick. Simple in line car type fuses are all they need to stop construction site fires.
However there is a reason why it is no longer used, and not solely cost, time has shown using a simple temperature controlled device in the main works better with less maintenance. I would think around 1985 was last time I topped up a dash pot, I seem to remember that transformer oil is cariogenic and items like motor starters and welding pods were always greasy with the oil on the outside, fire resistance fluid is still used, last time I saw a spill it caused the building to be closed down for a week for cleaning.
Over time I have found the simple fuse is still a good protective device, I have worked where machines were modified to reduce down time, one modification was to replace the contactor with a solid state device, however this produced two other problems, one was it could no longer be used for the E-Stop so we had to introduce an E-Stop contactor, and second we found from bitter experience when the heaters failed and went short circuit it would take out the solid state device every time in spite having MCB's fitted. The cure was to replace the MCB's with semi-conductor fuses, they would rupture faster than a MCB and would save the solid state device from failing.
I am not talking theory, it was shown in practice the fuse was a better protective device than the MCB. There is a problem mixing the MCB and the fuse, it is hard to get the discrimination required, back in 1980's simple bulbs had a fuse-able link built in so if when the bulb reached end of life there was ionisation in the bulb the link would rupture so the rest of the lights on the circuit would continue to operate as the link would rupture before the main fuse would blow. As fuses were replace in the distribution unit for MCB's it was found that the MCB was faster than the fuse link in the bulb, and when they failed with ionisation it would cause all lights on the circuit to fail rather than one single lamp.
However as time went on we stopped getting British made bulbs, and the imports often lacked the internal fuse, renewing the distribution box fuse became more regular so we had to go over to the MCB so it could be easy reset, it was nothing to do with MCB protecting better it was the problem with using inferior bulbs, and the cost of replacing the cartridge or uncertainty of the correct wire being used to replace the blown fuse wire which forced us to move over to the MCB. The MCB is often faster acting than the fuse, but it can also be slower acting, the MCB has a very defined point where thermal takes over from magnetic protection. The prospective short circuit current is extremely important, go under that critical value by just 1 amp and the time jumps from 0.01 seconds to 5 seconds. This is not a problem with the fuse. So as we went over to the MCB then we also had to carefully measure the PSCC or loop impedance just a little under or over the limit and the device failed.
Today we have many overload protective devices, including electronic used for motor protection built into inverters and soft starts, some only protect the device targeted, the inverter protects the motor, but not necessary the cable or personnel working around it. So we have to design, it is not a case of we always use a xyz, we have to consider what is required, we are then often required to sign paperwork to say how we have the skill to do that design and accept the responsibility for our actions. We are guided by the books which have charts to assist us, like BS7671, where it suggests the time which will be needed for the automatic disconnection of the supply should be within laid down limits.
We don't always get it right, I have seen many times specially with reduced low voltage where people have got it wrong, putting a 10A overload into the supply of a 240 to 55 - 0 - 55 volt transformer means the output will need to exceed 43.5 amp to earth before the input device will operate, yet the output is through a 16A socket. I would like to ban all yellow brick transformers they are dangerous devices and don't comply with regulations, at least until they start putting the over load device on the output. But it's not the fuse or overload at fault, it is the design of the yellow brick. Simple in line car type fuses are all they need to stop construction site fires.
