Fuses. Introduction. Measuring is knowing. Who chooses?

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1 Fuses Introduction. There are many theories going around about the necessity for protecting PDUs (Power Distribution Unit / multiple socket / socket, etc.). A fuse is by definition a Single Point of Failure (SpoF) and therefore undesirable in a data centre. However, there are situations in which a fuse is necessary, sometimes even obligatory. We cannot therefore dispose of them by saying we don t want it and we therefore don t need them. Schleifenbauer Products has attempted to lay things out for you and has made an overview of what you should know about fuses in a PDU. Fuses can be applied in all types of PDUs but are mandatory in 32A models with 16A outlets, to guarantee safety. Fuses are also used to limit the damage caused by errors to just one segment on a PDU. Because of the intelligent measurements in the Schleifenbauer PDU, power consumption can in any case be monitored and an alarm can be given if it escalates into the danger zone. Measuring is knowing. Fusing is a drastic safety method. If the current exceeds a certain norm for a certain time, then it is abruptly switched off and all the devices after the fuse become powerless. It is as though you were in the dark on the edge of an abyss. Schleifenbauer PDUs are fitted with power meters that accurately indicate the actual power consumption. This enables you to take timely action if power consumption rises too high. It is, however, not always possible to measure power at every sublevel and short-circuits cannot be predicted by ampere meters. Therefore, fuses despite the presence of meters are unavoidable and you should make an informed choice about their application. Who chooses? The responsibility for choosing whether to use fuses or not, or which fuses will be used, is at all times the end user s, as long as it remains within the legal framework. From the information below, it will be apparent that no single best solution exists. The best solution will depend on the specific situation in your data centre and can therefore differ for every client. Schleifenbauer Products offers you the possibility to choose exactly the fusing that appears to be best for your situation. That is why we customise our PDUs for each customer.

2 Why fuses? There are two reasons for installing fuses in a PDU: 1) safety: one outlet (plug) must be protected from overloading by maximally the nominal value of the outlet. This means that a PDU that is connected to a 32A feed and has outlets with a maximum load of 16A (such as earth, Wieland or C19), must be protected by fuses of maximally 16A (C13 is an exception because one C13 has a maximum load of 10A, but since they are usually grouped together, it is permissible to protect them likewise with 16A fuses). 2) isolation of errors: another reason for fusing is the isolation of errors to a certain segment. A PDU without protection will be completely switched off in an error situation. A PDU with protection can isolate the error to a limited segment of the PDU and thereby guarantee the continuity of the remaining equipment. Safety at various levels has been incorporated into the installation network. See the diagram below. You could view a PDU in the same way. They are often connected to a power circuit with its own protection (sometimes shared with several other PDUs). If an error occurs after 2, then it is desirable that only fuse 2 is activated and not 1, because then the rest of the PDU would also be switched off. This is called selectivity. This is one of the most important parameters that must be well understood in order to make a proper decision about the use of fuses. 2 1 main fuse There is selectivity if one fuse will act sooner than the previous (main) fuse. If, in the above case, 2 always reacts earlier than 1, then we call this 100% selectivity of 2 with respect to 1. In practice however, 2 is only selective with respect to 1 up to a certain short-circuit current and not beyond it. What do you want to protect? sub-devider board It is good to first determine what you want to protect: do you wish to avoid the risk of overloading, short-circuiting or both? Overloading: Overloading takes place if more current is used over the long term than is permitted for the components in question. The speed at which the fuse is activated depends on the type and specifications of the fuse. Because our PDUs are always fitted with over-rated wiring, they can withstand such overloads and the fuse does not need to activate quickly. For protection against overloading only, we therefore advise a simple thermal fuse on a bi-metal basis. When the fuse is activated, it can be reset by the users themselves by pressing in the protruding switch. Short-circuiting: A short-circuit is a direct contact between the phase and the neutral (or earth), by which a current is created that is equal to the maximum short-circuit value at that point. It is important to know this value in order to make a proper choice for a safety device. The short-circuit current is greatest close to the source (transformer or UPS) and then reduces due to damping and losses in the wiring and transition resistance between all the intervening links and safety devices. A good electrician or installer can make a calculation of all the short-circuit currents in the data centre. Safety switches provide protection to a certain maximum short-circuit current. If the short-circuit current after a safety device (fuse) is higher than the value on this safety device, the risk of electrical arcing arises in the safety device. This can lead to fires and the current to the error is not(!) interrupted. This is thus extremely undesirable.

3 Types of fuse In the table below, you will find an overview of the fuses that can occur in Schleifenbauer PDUs. type Regular fuse Automatic fuse Thermal breaker remarks Consists of a cartridge and a holder. Fusing against overloading and short-circuiting by means of physical (and chemical) operation. Only interrupts the phase. Needs current to operate (voltage drop mV) There are 2 types of holder: simple holders with a screw cap without indication or DIN rail holders (the same shape as MCBs) with a bayonet fitting and indication. Has to be replaced after an error. Can be replaced by a different type of fuse. High resistance to short-circuiting. Accelerated aging with changing loads (in-rush current) MCB (Miniature Circuit Breaker) Operates on the basis of thermal energy (overloading) and a magnetic relay (short-circuit). Uses slightly less energy than a fuse. Also optional interruption of ZERO. Resettable after error or manual switch-off. More vulnerable to breakdowns than regular fuses. Must be replaced after 3 5 short-circuits. The fuses cannot be adapted. Selectivity has to be established by means of calculations. Normal short-circuit resistance of 6kA, also available with 10kA. Thermal (bi-metal) protection against overloading. Resettable after error. Uses relatively little power (voltage drop 240mV). Switches only the phase. Does not react (too slow) to short-circuiting and is thus not selective. Low short-circuit resistance of 2kA. Small and cheap. What do you need to know? In order to make a decision regarding fuses in your PDU, you will have to make a number of matters clear. Below, you will find a list of terms occurring in the decision process, with a brief explanation. Selectivity: As previously stated, selectivity is one of the most important characteristics of a fuse. Selectivity is a number that indicates to what extent a fuse will protect you against short-circuiting before the previous fuse(s) is/are activated. Between fuses there is a legally applicable selectivity of 1.6. This means that a 16A fuse is selective with respect to a 32A fuse but not a 25A fuse ( is 25.6). The selectivity between MCBs or in a mixed environment is more difficult to establish. We then have to project the time-link graphs for both fuses over each other, so that it becomes clear at what short-circuit current which fuse will activate first. The MCBs are usually provided with an indication of their selectivity class. This number doesn t mean anything yet, however. You have to have a calculation of the short-circuit current made in every location in the data centre. Only then, if you know the technical data of the previously placed fuse, can you establish whether a fuse is selective with its predecessor in the chain. Short-circuit current: The short-circuit current is the maximum current that can be generated at a certain point in the electrical system. The shortcircuit current close to the transformer or the batteries of the UPS is many times higher than the current that can arise in a 19 cabinet. This has to do with the damping influences of the power cables and all the transition resistances in the system. From this, it follows that a rack close to distribution panel will have a higher short-circuit current than a rack tens of metres

4 further away. There are programs that can calculate short-circuit currents. However, we advise you to have your electrician make a comprehensive diagram in which all the short-circuit currents are noted. With this document you will be better able to take decisions with regard to fuses in a PDU. Absorbed power: A fuse absorbs power. Since a fuse always reacts thermally (sometimes also magnetically), it is logical that a certain amount of the electrical energy is used to develop heat. If it is much too hot (current too high) then the fuse activates. The absorbed power is often indicated by the voltage drop across a fuse. This voltage drop multiplied by the current (I) is the absorbed power. A fuse that is heavily loaded (<80% of Inominal) will absorb a few (3 5) Watts of power. An MCB absorbs a little less power than a fuse. In the case of several fuses in a PDU (often with thermal breakers) the number of fuses more or less cancels out the lower power per fuse: 10 breakers each of 1.5A take up about the same power as 1 breaker of 15A passing through it. Environmental temperature: The environmental temperature affects the nature of a fuse. The higher the temperature, the sooner a fuse will react to overloading. You should take this into account if the PDU is mounted directly in the exhaust of the rack equipment. Maximum throughput energy: A fuse allows a certain amount of energy through before power transmission is interrupted. This energy is the throughput energy and can be calculated using the following formula: I 2 t (the short-circuit current squared times the time) this energy must be small enough so that no fires or damage can occur to the equipment or wiring. Usually, a regular fuse has a much smaller throughput energy than an MCB because the melting process is faster than the thermal-mechanical process in an MCB. Short-circuit resistance: This term indicates to what extent a safety device is resistant to short-circuit currents. A normal MCB usually has a shortcircuit resistance of 6000A (6kA). It is capable of switching loads up to 6kA. When the actual short-circuit current is higher than 6kA it means that there are chances of serious damage to the MCB, fire or the occurrence of a persistent electric arc, which allows the current through to the following wiring and will almost certainly cause a fire as well, are high. It is therefore very important to know what the short-circuit current is in the spot where you want to put a fuse. Thermal fuses usually have a higher short-circuit resistance: when the wire has melted, there is a great distance between the phase and the descending wiring. In a sand-filled fuse, the melting silver wire reacts with the silicon in the fuse and a type of glass wire with infinite resistance is created. Flexibility: Once an MCB or thermal fuse is built in, it cannot be replaced without removing the whole PDU. If it breaks down or wears out, the PDU has to be dismantled. A fuse can always be replaced without dismantling. A danger with this is that they could also be replaced with a fuse of the wrong value. A fixed fuse does not have this risk. Reliability: MCBs and thermal fuses work on a mechanical principle. The regular fuse works on the basis of physical and chemical reactions (melting and reacting). Both principles have advantages and disadvantages. MCBs have more than 100 components, which is in itself more vulnerable than a single silver (or any alloy) wire. In normal circumstances, the MCB has already proven itself as a reliable instrument. On the other hand, if an MCB has undergone several (3 5) short-circuit situations with high short-circuit currents, it must be replaced. Moreover, an MCB can fail in two ways: through long-term lack of use it can sometimes fail to switch off when necessary and sometimes, it will not switch back on after switching off. A regular fuse seems easier: it either works or it doesn t... however, a regular fuse can be weakened by brief power surges that create small melted points in the metal wire, due to which the characteristics of the fuse can change. Regular fuses also have more problems with aging than MCBs. Actually, regular fuses should be replaced every 2 3 years.

5 Ease of use: MCBs are unmistakably easier to use. A breakdown is immediately visible (even if the manual switch is fitted with a lock pin) and after an error, it can immediately be activated. MCBs must be replaced after 3 5 short-circuit situations. Therefore, an administrative system that registers which MCBs have had errors also has to be implemented. A regular fuse must be replaced after one error. The user must know where they are located. A logistics system has to be implemented for this. There is also the possibility that the fuse will be replaced by the wrong one, through which dangerous or undesirable situations could possibly arise. A defective cartridge can be replaced without any fuss and the holders seldom or never break down. MCBs, on the other hand, can break down due to their complexity or stick, due to which they will no longer switch when necessary. If an MCB breaks down, the whole PDU has to be switched off and repaired. The choice: The choice for a certain safety switch is ultimately the client s responsibility. This document is exclusively intended to provide you with information so that you can make the choice with more substantive knowledge. Two choices will have to be made: which fuse and how many? If protection from overloading only is sufficient for you, then a thermal breaker is a good and inexpensive option. You will hereby fulfil the legal requirements with regard to the protection of outlets. If you wish to protect against both overloading and short-circuiting, then the decision is a bit more complicated: you can choose from MCBs (installation of automatic) or regular fuses. The choice depends on the desired selectivity. It could follow from this that a regular fuse is preferable to an automatic one. The disadvantage of regular fuses is that the manager of the data centre has to implement a logistics system for the availability of the correct (!) fuses. A regular fuse can also easily be replaced by the wrong one, a risk that does not occur with MCBs. A disadvantage of MCBs is that they have to be replaced after a number of short-circuits. This means that the whole PDU has to be dismantled. MCBs contain more than 100 mechanical components, which makes them more vulnerable than regular fuses. These, however, have the disadvantage that they age over time which affects the values of the fuse and actually (depending on the loading) should be replaced every few years. In short, ideal protection does not (yet) exist. Indeed, there are developments in the field of electronic safety switches, but they are not yet available and will probably be much more expensive than current solutions. To make the proper choice you must therefore make sure that you have as much information as possible about your infrastructure (short-circuit currents, selectivity, etc.) and that you understand the consequences of your choice. The number of fuses you will use will depend on a number of situations: If there is a 32A feed present, you will have to use at least two 16A fuses, otherwise you can only use 16A of the 32A present. However, you can also opt to install more than 2 fuses, to limit errors in the system to as small an area as possible. If you also wish to apply this for short-circuiting, then you must pay close attention to the selectivity of the chosen fuse. A thermal breaker, for example, is not selective for short-circuiting. Regular fuses with MCB-shaped fuse holders and MCBs both take up a great deal of space.

6 MCB (automatic) Regular fuse advantages -Reusable -Optionally switch both phase and neutral -Can be used as a switch -More practical than regular fuses -Use (slightly) less power -After an error a new cartridge -More selective than MCBs -Allow less energy throughput -Can be replaced by a cartridge with different characteristics disadvantages -Have to be replaced after a number (3 to 5) of errors (short-circuits), depending on the shortcircuit current (how should it be administered?) -Less selectivity than regular fuses -Higher energy throughput than regular fuses -Take up a relatively large amount of space -Are relatively vulnerable -The PDU has to be dismantled with replacement -Not reusable -Logistics system needed for spare cartridges -Can conceal wrong cartridge -Age more quickly Conclusion. If you wish to protect your system against overloading and/or short-circuiting, you will have to analyse the chances of this occurring and the costs incurred if it happens. Overloading can be largely prevented by the readings on the ampere meters but short-circuits cannot be predicted. If the risks are too high, you will have to install fuses to limit the errors. The proper choice for the type of fuse can only take place if you have a detailed picture of the electrical system in your data centre. Schleifenbauer then makes a PDU with the fuses that are most suitable for your situation. Interesting background information on the subject of fuses can be found on the web site of EATON HOLEC: in the section vaktechniek (professional technology). Here you will find readable documents that go further into the technical details of safety devices. (Dutch only, sorry)