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Surge Protective Devices and High Ra.

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What adverse effects can be expected under over Voltage fault conditions, when an S.P.D. is required to operate  on a TT system with a high Ra.

Longer duration fault transient Voltages?

Higher fault Voltages supplied to vulnerable equipment?

"3.2.2 Time/energy relationship The current must flow for sufficient time to deposit enough energy within electronic components to cause damage – commonly the melting down of some part of the device. 3.3 Surge protection devices (SPDs) – how they work Surge protection devices (see the glossary – Appendix B for other commonly-used terms for these) limit the transient voltage to a level which is safe for the equipment they protect by conducting the large surge current safely to ground through the earth conductor system. Current flows past, rather than through, the protected equipment and the SPD thereby diverts the surge (see figure 8). The SPD limits both common and difference mode voltages to the equipment."

Z.

  • 0

    I think you need to draw a distinction between

    a) very high but short duration overvoltages, that trigger the SPD into its low resistance state, such as pulses of a kilovolt or two for microseconds, perhaps clipped to several hundred volts by allowing hundreds of amps to pass through the SPD - but only for microseconds remember.

    b) overvoltages of the kind where the neutral falls off and the 230V rises to 300-400, but potentially for seconds or minutes.

    In the second case the SPD will hopefully not trigger, but if it does then there is likely to be quite a lot of damage as although the currents are lower, the fault duration is too long.

    Mike.

  • 0 in reply to mapj1

    Thanks Mike.

    Z.

  • 0 in reply to mapj1

    Thinking further today I was looking at Figure 16A3. I was particularly interested in item 3a, the gas discharge tube connected between N and E. I do not know how the operation of the 3a item is affected by the Ra of the system. Say a, Ra is 50 Ohms, or b, 100 Ohms, or c, 500 Ohms or d, 1000 Ohms or e, 1500 Ohms .With a very high transient spike how would the operation of 3a be affected?

    Z.

  • 0

    Thinking further today I was looking at Figure 16A3. I was particularly interested in item 3a, the gas discharge tube connected between N and E. I do not know how the operation of the 3a item is affected by the Ra of the system. Say a, Ra is 50 Ohms, or b, 100 Ohms, or c, 500 Ohms or d, 1000 Ohms or e, 1500 Ohms .With a very high transient spike how would the operation of 3a be affected?

    A high Ra will make some difference, but not have much of an effect I would have thought. The device will sense the overvoltage and start conducting as normal - the current flow to Earth may be much reduced but the resulting voltage between the SPDs terminals should be much the same as for a TN system (or if anything perhaps a little lower due to the reduced current). The voltage on both terminals might be a lot higher above true Earth than on a TN system - but the protected devices (downstream) don't see that - they should only be exposed to the voltage difference between the SPD's terminals (L on one end and PE (or N) on the other - which is why we have to keep the connections between the SPD and the point it connects to the circuit it protects short.

       - Andy.

  • 0

    well the Ra /Ze /Zs are not really what you need here - for the leading edge of short transients there could be as much current flowing into the capacitance of the wiring acting as one plate and the rest of the universe as the other, as there will be actually flowing into or out of the electrode(s).

    As a first cut rule of thumb, consider the electrode as some resistance to terra-firma, in series with a micro-henry or so per metre of wire to get to the electrode frome where you are measuring,  and the whole lot in shunt with perhaps a few hundreds of pF or nanofarads depending on the size of the installation, between every wire and the ground beneath it.
    (To do it properly involves looking at the wires over ground as interconnected and coupled transmission lines and solving for velocity factor and characteristic impedance. I suspect most folk prefer the rule of thumb.)

    At 50Hz the resistance dominates, and all the OSG approximtions are good, but for short duration events, that is no true and the L-C transmission line effects can no longer be neglected as small.

    If the electrode is 'far away' compared to the time of the transient then even if it had a fantastically  low impedance no current would flow, as the electrons would not get there in time.

    Having said that, the L, N and E will all bounce together once the MOV and gas discharge have entered their conducting state. This stops sensitive electronics 'doing the splits' being between the lines.That is all the SPD guarantees. What voltage the lines are at relative to something far away is not controlled - they may all bounce up to a few kV relative to another earth electrode if you could bring the voltage of one in from far away without bonding it.

    Note also that for the same reasons we too are not affected by short pulses in the way that we are by steady voltage - the 10kV kick of an electric fence may exercise your 'vocabulary of opportunity', but is nothing like as deadly as the same voltage  (and current) would be persisting for a cycle or two of 50Hz instead of being all over in a hundred microseconds or so.

    Mike.

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