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Regulation 534.4.4.6

MG
Greetings to the forum.

Can someone clarify this for me. Regulation 534.4.4.6 says:
"In general, the short-circuit current rating ISCCR of an SPD, as stated by the manufacturer, shall not be lower than the maximum prospective short-circuit current at the connection points of the SPD assembly"

Then Note 1 below the regulation says:
"The SPD alone, or as an assembly in conjunction with its disconnector and/or overcurrent protective device (OCPD), is required to withstand the short-circuit current rating ISCCR as stated by the manufacturer. This value is defined and tested according to BS EN 61643-11, in conjunction with the disconnector and/or OCPD as stated by the manufacturer"


I understand that ISCCR (in kA) is a short circuit withstand capability value given by the manufacturer on the condition the maximum upstream protective device rating is not exceeded.


If the upstream SPD protective breaker/fuse has a breaking capacity higher than the system fault level, can an SPD with a ISCCR lower than the system fault level be used? I am confused because Note 1 says either the SPD alone or SPD+Disconnector as an assembly has to withstand the system fault level.


Reason for all this scenario is because I have come across an SPD with short circuit withstand capability of 25kA protected upstream by a breaker with Icu of 50kA is specified on a system with 37kA fault level.


Thanks,

M.


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  • 0

    MG:

    ...protected upstream by a breaker with Icu of 50kA...




    No 'seemed to say' about it. I can see two possible explanations.

    1. The SPD is at the end of a significant cable length so the fault level at the SPD is significantly lower than the 37kA at the upstream breaker (rather unlikely), or

    2. The designer got it wrong.

    (There is a possibility of 3. The upstream breaker provides limitation of let through energy, but while I have seen breakers of that type they generally have a much higher Icu so I have discounted this)

    I am hoping that someone can come up with a further alternative which I haven't thought of.

    Alasdair

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  • 0

    MG:

    ...protected upstream by a breaker with Icu of 50kA...




    No 'seemed to say' about it. I can see two possible explanations.

    1. The SPD is at the end of a significant cable length so the fault level at the SPD is significantly lower than the 37kA at the upstream breaker (rather unlikely), or

    2. The designer got it wrong.

    (There is a possibility of 3. The upstream breaker provides limitation of let through energy, but while I have seen breakers of that type they generally have a much higher Icu so I have discounted this)

    I am hoping that someone can come up with a further alternative which I haven't thought of.

    Alasdair

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