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This month, we need your help as we try to understand better what are the applications where LV fuses are much more adapted than LV circuit-breakers because CBs are technically not fully adapted to the requirements.
We already have in mind the following applications where we believe LV fuses characteristics are more suitable than the characteristics of most existing LV circuit-breaker offers:
1. Protection of speed drives
2. Protection against short-circuit when CB characteristics are too short (Voltage >=500V, High Icc level)
3. Protection of capacitor banks
4. Protection in DC applications (UPS, PV inverters …)==> please tell us if you agree with this list
==> if you think about other applications, tell us the ones where fuses are technically more adapted than CBThanks for your help !
Moderators teamDiscrimination is easier to achieve with fuses than circuit breakers especially in small sizes. At a small installation you can get three layers of discrimination with 50A, 32A and 16A fuses or even 40A, 25A , 10A. You can’t do this with breakers. Often these installations have a fairly small diesel generator and the top level of protection should be kept small enough to be cleared by the available current from the alternator. If there is a UPS, that complicates matters further. Fuses are often the only option to ensure a short circuit in a sub-circuit does not take out theb whole site (it has happened)
Circuit breaker is acceptable for small or large M/C, please select the correct specification and rating according to M/C application.
HRC fuse are only very use full and safe in control schemes.
Agree with holwa on discrimination for small ratings . Be careful to at least have a gap in the fuse ratings series . For example, you may loose discrimination with 10A, 16A, 25A . In this case, the three ratings are too close.
LV fuses are more suitable for Short circuit protection where speed of arc clearance is too small by the Circuit breakers or in other words Fuses are the only option when you want to limit the short circuit currents to not more than 1/2 cycles or even less. A typical example is given below
An electrical Power system employing 8 Alternators of 2000KVA @690V 3 Ph 60 Hz and Xd” of 20%. is required to feed electrical Variable speed drive. Unfortunately, the availability of drives in market is limited to a maximum KA of 65KA. Therefore it is not possible to feed VSD directly from 690V Switchboard (fed from 8 Alternators). How do we solve this problem.
1. By splitting the 690V bus in to two Sections, each supplied by 4 alternators
2. Bus coupling is achieved by employing a Short circuit limiter which limits the short circuit flow between two buses to 10KA
By employing above arrangement, we can limit the shirt circuit level of 690V bus.Fuses are vunerable at low fault levels (they take more time to operate than CBs)
Circuit breakers are vulnerable at high fault levels (They take more time to operate than fuses)
Fast acting fuses are better option when we require to couple a low Short circuit rated network to an infinite bus or network with high short circuit rating. IS limiting fuses are also desirable when coupling two or more LV networks with moderately high short circuit rating where the combined Short circuit rating of the coupled network exceeds technically feasible values. In such cases by inserting fuses with proper clipping current characteristics, we can limit the short circuit rating of the coupled networks.
Dear Mr. Anilsabaji ;
First, as you said the VSD can withstand up to “ 65kA ”, but I want to add that this value is depending on the VSD’s Type and the manufactures.
About your example “ 8 Alternators ” I have the following comments :
1 – As we know all that many CB manufactures produce a special rang of CB called “ Limiter CB ” that can limit the Short Circuit Currents, and also they added in the catalogs some special tables containing the “ Upstream & Downstream Values ” of the short circuit currents for each “ Limiter CB ” as a guide for the users, so, we can use them where we need to limit the short circuit currents even to a value smaller than “ 10kA ”.
2 – As I understood that each Alternator is “ 2000kVA, 690V, 60Hz, X”d=20% ”, and by a simple calculation we know that exactly at the connection points “ Isc3max = 9.23kA ”, and by using the necessary cables ( only 15meters ) it drops to “ ≈ 9kA ”, that means by splitting these 8 alternators into 2 groups of “ 4 ”, the Short Circuit Current value at BB will be “ ≈ 36kA ”, so, we don’t have any problem for the big VSD, and for the small the problem can be solved by using a Limiter CB.
3 – Also we know all the there are some weakness points of Fuses, like : 1 of 3 fuses blown that are using to protect an Outgoing, where to know it we should use a special fuses equipped with “ Striker ” with also a special base of fuse holder equipped with “ Auxiliary contact ” by which we will have a signal once happen.
4 – Using the Fuses to protect circuits oblige to have Fuses as spare parts from all installed kinds and ratings to be able to replace any blown one.
Hello friends,
I would like to say, the lv fuse’s breaking capacity can be significantly increased and current-limitation achieved by means of a granular filler (usually quartz sand) embedding the fuse element. Some fuses have indicators or strikers, tripped by the recovery voltage, to indicate whether the fuse has operated and to allow for easier detection of faulted circuits.
Although the basic design of fuses has not changed much in many years, the evolving engineering methods and modern materials sciences assist to keep improving the performance of fuses.
Regards
@anilsabaji said:
Fast acting fuses are better option when we require to couple a low Short circuit rated network to an infinite bus or network with high short circuit rating. IS limiting fuses are also desirable when coupling two or more LV networks with moderately high short circuit rating where the combined Short circuit rating of the coupled network exceeds technically feasible values. In such cases by inserting fuses with proper clipping current characteristics, we can limit the short circuit rating of the coupled networks.
agreed with views of Anil Sabaji.
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