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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.

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  • 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.

Reply
  • 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.

Children
  • 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.

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