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How many SPDs?

Chris Pearson

We have talked a bit recently about positioning SPDs, but not how many need to be fitted.

534.4.4.2 says, "If the distance between the SPD and equipment to be protected (protective distance) is greater than 10 m, oscillations could lead to a voltage at the equipment terminals of up to twice the SPD's voltage protection level. Consideration shall be given to the provision of additional coordinated SPDs, closer to the equipment, or the selection of SPDs with a lower voltage protection level."

Earlier we have at 534.4.1.4, "Where the installation of SPDs is required by Section 443 [and no LPS is required], Type 2 SPDs shall be installed as close as possible to the origin of the electrical installation."

The scenario is main DB at the intake in the garage serving circuits in the outbuildings (and an EV at some stage in the future) with a distribution circuit to the house. Nothing in the garage is likely to require protection from surges save perhaps for PIR controlled external lighting. The distribution circuit is > 10 m.

The house DB will be changed soon, so adding SPD will not cost much in time or money, but adding it at the intake would be tricky.

So my question is will SPD in the house DB suffice? Or will it not work unless there is also one at the origin (suitably cascaded)?

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  • 0


    Maybe with a Up as low as this, most equipment will have a Uw sufficiently high even if the oscillation was x2. 

  • 0 in reply to lyledunn

    Thank you. I appreciate that such a device may be sufficient for a long installation, but that's not answering my question. It's all about cost-effectiveness.

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  • 0 in reply to lyledunn

    Thank you. I appreciate that such a device may be sufficient for a long installation, but that's not answering my question. It's all about cost-effectiveness.

Children
  • 0 in reply to Chris Pearson

    If there is nothing to protect from surge at the intake position, then there is no point putting a SPD there IMO. It will give you better protection of downstream circuits, but, if they do work correctly, the SPD at the CU should do the same job. This is another area where little actual data is available to show they work, and show what thye do protect.

    However, the ones at the intake are usually a combined Type 1 and 2, whereas the consumer unit ones are Type 2 only.

    If you put  a combined 1 & 2 in , or close to, the CU, then I cannot see a problem, as it'll be protected, as much as practically possible. I would tend toward the standalone isloator/SPD combined units before the CU.it gives just a bit more copper between the SPD and CU, and is far easier to change in the future if ever one of the modules fail.

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