High Voltage: circuit breaker is the best!

ringmaster outdoorEvery time I hear or read something about high voltage fuses, I’m convinced that15 years ago in the UK, we made the right choice by moving away from high voltage fuses to circuit-breakers.

If we take the first argument I read in a previous article, that a fuse-switch is a cost effective solution, this is in fact a myth. In the past it was true due to the physical size of the old circuit-breakers.  Now, compact RMUs with dedicated circuit breakers and self-powered relays have the same life time costs as fuse-switch solutions and have far more useful features.

The main, or should I say only, technical argument for the fuse claiming high fault clearance speed is biased.  It is extremely unlikely that a 3-phase short circuit would occur inside the transformer without forewarning by a low magnitude fault, for example an inter-turn fault.  In this example a fuse would be unable to clear the fault, whereas a circuit-breaker works perfectly on something that is similar to an overload.  This is even truer in other countries, on networks having either isolated or resonant neutral systems.  Also, even on high magnitude faults, circuit-breakers function well, clearing faults quickly enough.

Self-powered relays are simple and easy to set.  Also, settings can be changed as the installation evolves.  This removes doubt over the selection of the proper fuse when replacement is needed either after a fault or for other reasons.

Upstream and downstream coordination is much easier with circuit-breakers.  Network management is better facilitated.  In case of a fault on the network, the overall time, including fault finding up to re-energising is reduced with circuit-breakers compared to the use of fuse-switch solutions.

More arguments exist but, I think it is enough to understand why there is a trend towards the use of circuit-breakers instead of fuse-switches.

Richard Douglas


  • Fuse disconnectors are safer during maintenance as there is no visible isolation in Circuit breakers. more over circuit breakers deteriorate as the number of fault occurence increase. In my openion fuse disconnectors are better where dedicated maintenance teams are available.

    • I dont think so. “None visible isolation” is very old fashion. Now, with the new compact RMUs that have the rays, tracks, locks… you must take a lot of actions to ensure your safety before you can take the circuit-breaker outside for maintenance. If you missed only a supper small action, the lock could not open to allow you work/carry on anything.

  • CB is more suitable for general use without regarding the cost.

    Who can tell me which brand are chipper ?

  • Oh, tell me the brand name you want to compare with, so I can suggest you some other brands might cheaper.

  • I agree completely with the original post. The move away from oil insulated switchgear was a good one.

    I can highly recommend the Merlin Gerin Ringmaster range of SF6 CB’s and switchgear.

  • I also preferred to use circuit breaker if money is not an issue. Circuit breaker is far better than the fuses.

  • I also preferred to use circuit breaker instead of fuses. If money is not an issue, circuit breaker is most suitable.

  • I was called to an HV fault yesterday. to cut a long story short we found an 800kVA transformer is showing an HV winding resistance of 14k ohms to earth on the delta winding. We took the lid off expecting to smell somthing but it was fine. Oil from an adjacent transformer looked and smelt exactly the same. No evidence of anything in the tank.
    The fault caused 90 amp (11kV) fuses to operate TWICE. There is IDMT protection downstream of the fuses which is a bit unusual bit no flags had operated.
    At present, pending full maintenance of the switchgear etc etc we suspect that the fault was cleared by the fuses “immediatly” allowing very little energy to flow and therefore show no signs of damage.

    I did have a nasty moment though… I ended up closing an earth switch to earth on the DNO’s infeed due to incorrectly labelled switchgear. ( no comments please it was my responsibility to sort it) but the switch operated perfectly and with no drama, in fact I did not realise I had earthed the circuit until many hours later. The poor old switchfuse again operated and I suspeect we will find all 3 fuses operated but the speed of operation and the limitation of fault energy was quite impressive. When we open the earth switch I will report on the state inside.
    Learning Points:-
    1. Never operate a switch unless you have a single line diagram, and dont earth a live circuit!!!!
    2. Ensure that fuses do grade with the IDMT
    3. Don’t ignore fuses, they do limit fault current
    4. They may allow fault current to flow but by restricting the damage the fault may self clear (temporarily)
    5. If possible get someone else to operate the switch!!!!!!!!!

  • Fuses have the only advantage over circuit breakers in the fact that, irrespective of the fault level, they limit the let-through energy to a predictable level. This is not the case with the circuit breakers which, irrespective of the sophistication of the protection relays, can only open within a finite time that is an inherent function of the mechanical inertia of the switch contacs.
    In all other respects the circuit breakers are superior and the price difference, if any, should not be the reason for preferring fuses.
    I have been specifying CBs only for at least 15 years and never had reasons to regret it.

  • Circuit breakers can operate extremely quickly if they are connected to instantaneous protection such as unit protection REF, Solkor etc. We had a 33kV circuit which was energised for 3 days before the missing lug and boot finaly flashed over! As it was within a unit protection zone the fault energy was really very small even though the FL was around 400MVA We used a little bit of emery cloth and fitted a lug and all was well. If this had been in an IDMT protected zone we would probably have had to change the switchgear. A fuse would have had the same limiting effect probably.

  • Everything is fine unless something goes wrong….so it is from failures which we learn. Selecting the appropriate circuit protection device may be a bit of a gray area, I admit. An engineer should always be consulted before subsituting CBs for fuses in an installation. That engineer will be familiar with the pros and cons of circuit protection equipment.

    A fuse is not a moving part, and cannot jam, freeze or corrode to failure. An oil filled fuse is sensitive to vibration and impact though…and the resultant spilled oil might well be an issue. Ice, frost, and heat won’t affect fuses as much as they affect CBs.

    Circuit breakers are often used as switches. When you leave the installation, you have little control over what the user does with it. Example…factory lighting rarely has separate switches…the maintenance crew actuates the CBs as if they were switches. Expensive switches. Constant use may degrade the Circuit Breaker function and there is not a darned thing you can do about it. On the other hand, for fault finding purposes on complex systems such as airplanes, CBs are much more useful since you can isolate circuits with a quick flip of the breaker. Why not pull a fuse? Well, if you could guarantee that human error won’t happen, then fuses would be the way to go….but a CB cuts the very human possibility of replacing a fuse with a wrong one right out of the equation.

    A Circuit breaker can weld its contacts and jam closed. Similarly, frost and ice can cause a CB to jam in the closed position…a potentially dangerous situation. A circuit breaker can also “weaken” over time. A circuit breaker is a moving part, and any moving part can fail. Nothing so annoying as searching for the intermittent fault and discovering that the problem is in the circuit breaker. Happens a lot with hot tubs and spas!

    Standard circuit breakers are not usually a problem, GFI’s and high voltage stuff can be a source of snags…fault finding procedures built upon fuses may overlook these delicate components. GFI’s are notorious for failing, fortunately they are designed to fail safe. Nobody I know has ever built a fuse based GFI…(insert wry grin here) but it can be hard to tell the customer that his brand new two hundred dollar GFI just went weak on him.

  • we always use General Electric circuit breakers at home because it is very reliable~’`

  • I install hot tubs and would always recommend to my customers to use a circuit breaker and not the fuse option and although they may cost more at the end of the day circuit breakers are safer when water is near by

  • Circuit breakers are always preferred over fuses in all new construction jobs my company works on.

  • circuit breaker do the safe of system normal & abnormal condition. you must take a lot of actions to ensure your safety before you can take the circuit-breaker outside for maintenance like earthed the breaker & carry on anything.

  • cct breakers are far much better than fuses.Theres is no need of stocking fuses in shelves

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