Whenever transitioning from a preceding supply cable protected by a higher current circuit breaker or fuse, I always fuse down before connecting the appropriate lower gauge wire.
What I wonder is then how such distribution blocks like these Weidmüller AWPD 35 4X6/6X2.5 are safe:
What I wonder is then how such distribution blocks like these Weidmüller AWPD 35 4X6/6X2.5 are safe:
They don't appear to be any different to thousands of bus-bar chambers at 400 A with 16 mm2 cable off the bar to the switch-fuse adjacent.
They rely on the integrity of the installer to provide a suitable protective device on the outgoing circuit.
Or, if you're looking at DNO world, there may be a 300 mm2 cable backed up with a 400 A fuse with 35 mm2 connected to it! The cable is protected from overload by the 80 A fuse in your cut-out (see 433.2).
Smaller conductors do in general require a smaller size fuse. but exceptions exist, for example if the larger conductor is very over sized for voltage drop reasons.
Or when over current protection is provided downstream. Example 35 mm cable on a 100 amp fuse, then several low current circuits in 2.5 mm cable on 16 amp fuses.
If the 2.5mm cables are short then you do not need 35mm cable into the holders for 16 amp fuses.
What I wonder is then how such distribution blocks like these Weidmüller AWPD 35 4X6/6X2.5 are safe:
Such blocks are often used as Neutral commoning blocks - in such cases the smaller conductors will be protected by the OPDs on the corresponding L conductors (ignoring faults between conductors of differing circuits, which is usually the case).
Where they're used on Line conductors, there will usually be downstream overcurrent protection which will protect the smaller conductors from overload and fault protection is either provided by an upstream device (in general an OPD will provide fault protection to much smaller conductors than it could provide overload protection for), or fault protection is omitted (most standards have provision for that - e.g. where the connections are short, precaution are taken to minimise the chances of a fault occurring, and precautions are taken to mitigate the results of a fault - e.g. to contain any resulting overheating or fire).
MJ, what do you mean by “safe”? Having decided that you can begin to understand what you are asking. Does safe consider the following, and if so why:
Downstream load characteristics
Likelihood of a particular fault
Corresponding fault current
Catoragise damage from the fault, and consequences
Lets assume the 35mm comes in from a 200A MCCB, and the outgoing small circuits are bits of control electronics as an example. Fill in the list and then you will see why it may not be necessary to “fuse down” everything. Please reply with your thoughts.
Whenever transitioning from a preceding supply cable protected by a higher current circuit breaker or fuse, I always fuse down before connecting the appropriate lower gauge wire.
What I wonder is then how such distribution blocks like these Weidmüller AWPD 35 4X6/6X2.5 are safe:
35mm² to 2.5mm²
Fusing down to protect a cable from overload is normally a wise thing to do. This will protect a cable from getting too hot and being damaged if a large overload current occurs. But if the load is fixed in value, say like a single heater, it can not overload a cable as it uses a fixed amount of current. So in some instances we are just concerned about a large fault current, which is different to an overload current.
If a load is of undetermined value, say by supplying sockets, anything may be plugged into those sockets and we would be wise to provide overload, or overcurrent protection for the supply cables. We do this with a ring final circuit protected by a 32 Amp device.
But larger cables supplied from a bus-bar chamber will be protected by a fuse or circuit breaker down stream of the connection to the bus-bars, even if the cable reduction occurs before the downstream fuse or M.C.B. The cables are protected against overload as if an excess current flows the fuse will blow, or the circuit breaker will open, thus protecting the upstream cables.
Conversely, most things are 'safe' if used in a properly planned and designed manner. As others have said - several applications where this can be ‘safe’ - but also lots of ways it could be used to generate an ‘unsafe’ condition.
Zoomup: Fusing down to protect a cable from overload is normally a wise thing to do. This will protect a cable from getting too hot and being damaged if a large overload current occurs. But if the load is fixed in value, say like a single heater, it can not overload a cable as it uses a fixed amount of current. So in some instances we are just concerned about a large fault current, which is different to an overload current.
If a load is of undetermined value, say by supplying sockets, anything may be plugged into those sockets and we would be wise to provide overload, or overcurrent protection for the supply cables. We do this with a ring final circuit protected by a 32 Amp device.
But larger cables supplied from a bus-bar chamber will be protected by a fuse or circuit breaker down stream of the connection to the bus-bars, even if the cable reduction occurs before the downstream fuse or M.C.B. The cables are protected against overload as if an excess current flows the fuse will blow, or the circuit breaker will open, thus protecting the upstream cables.
Z.
Precisely, my thinking is that fusing down should protect lower gauge wire feeding other circuits. In the same way when carrying out a PAT, you would typically replace a plug fuse attached to a figure 8 flex for 5A or 3A.
Zoomup: Fusing down to protect a cable from overload is normally a wise thing to do. This will protect a cable from getting too hot and being damaged if a large overload current occurs. But if the load is fixed in value, say like a single heater, it can not overload a cable as it uses a fixed amount of current. So in some instances we are just concerned about a large fault current, which is different to an overload current.
If a load is of undetermined value, say by supplying sockets, anything may be plugged into those sockets and we would be wise to provide overload, or overcurrent protection for the supply cables. We do this with a ring final circuit protected by a 32 Amp device.
But larger cables supplied from a bus-bar chamber will be protected by a fuse or circuit breaker down stream of the connection to the bus-bars, even if the cable reduction occurs before the downstream fuse or M.C.B. The cables are protected against overload as if an excess current flows the fuse will blow, or the circuit breaker will open, thus protecting the upstream cables.
Z.
Precisely, my thinking is that fusing down should protect lower gauge wire feeding other circuits. In the same way when carrying out a PAT, you would typically replace a plug fuse attached to a figure 8 flex for 5A or 3A.
I hope that the figure 8 flex has both insulated and sheathed conductors. I got caught out many years ago by putting 5 Amp fuses in the 13 Amp plug for our vacuum cleaner, based on its Wattage and the flex size. Then they kept blowing on startup. I had not catered for startup inrush motor currents. There again, I was only young then.
Precisely, my thinking is that fusing down should protect lower gauge wire feeding other circuits. In the same way when carrying out a PAT, you would typically replace a plug fuse attached to a figure 8 flex for 5A or 3A.
Keep in mind that almost all of the rest of the world manages perfectly well with unfused plugs - so there appliance leads are protected by faults only by the MCB or fuse back at the distribution board - typically 15A, 16A or 20A. Once you understand that protecting conductors from fault currents is quite a different proposition from protecting the same conductor from overload, it all becomes a lot clearer.
