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BS7671 551.7.2 Customer Limitation Scheme some thoughts

Ian Cheek

We wait with breath held for the amendment 4. Does it expand on the notes offered up during the consultation period, or leave it for industry to decide what is meant by, and hoe to implement a Customer Limitation Scheme.

In the past those around me have stated “we can just use G100”, but with no further statement on how G100 would work for our installation. While the concept is sound, limit the Solar to ensure the Ina is not exceeded, But how, the same timings 15seconds, 3 times in 24hours….

We think of Limitation and associated with G100, as a system that will cut off/reduce the Solar supply. Could it be by design instead, understanding the characteristics of the installation and see where the current flows. Current supplied from either end of a Bus bar, by the time they meet the will the sum exceed Ina.

15 Seconds above Ina, then maintaining Ina. The thermal equivalent will remain above Ina, diminishing over time but never reaching Ina, until the current falls below Ina. Do we need to set a time. If the overload is cleared by the OCPD as its designed to, the time in overload and the time returned to normal (lower than Ina) will give a thermal equivalent below Ina over time. The 15seconds becoming minutes, do we need a CLS or does the design work naturally.

Initially investigations to see if this Overload of Ina was a thing, as everyone was saying “it will never happen”. Quickly these investigations showed that it could, but they also showed examples where it couldn’t.

While there is only one catch all solution, which is currently defined in 551.7.2 Ina ≥ In + Igs. There are solutions out there that can meet the “as safe or safer”. They just need time and work, an Electrical Designer is unable to just look at a SLD as Yes or No.

Below while I’m unable to give an equation to get a Yes/No, below is my order of investigation work.

Establish Max Demand

    ½ hour data, Assumed Load

Establish Ieff

    T1, the length of time overload can exists without effecting Long duration Ina

Establish Overload Risk

   Distribution of current during overload, conditions Ina is not exceeded.

   Identify any masked overloads by the solar supply.

Establish actions to be taken by solar during overload conditions.

    Minimise the need to reduce/remove the Solar production.

    Through control of Solar Supply

    Through design

Maintain safe of safer than BS7671 amd 3 551.7.2

 

There is no guarantee this investigation will produce a desired result, it may just prove for the installation you are looking at Ina ≥ In + Igs is the solution.

If it does produce a solution by design, or the “G100” with a correct set of parameters for your installation. There will be cost savings for the disruption of the installation and the associated additional hardware.

Is it work worth doing, yes. Even if it just proves Ina ≥ In + Igs is the solution, then answer is known. The Design can progress passed all the “what if meetings that tend to stall all good projects”.

 

Remember Ina ≥ In + Igs will always work.

 

  • Ian Cheek said:
    In the past those around me have stated “we can just use G100”, but with no further statement on how G100 would work for our installation. While the concept is sound, limit the Solar to ensure the Ina is not exceeded, But how, the same timings 15seconds, 3 times in 24hours….

    Some experts in the industry would say that this is, strictly, an 'intended departure' from BS 7671:2018+A2:2022, and also BS 7671:2018+A2:2022+A3:2024, that has to be declared. See https://electrical.theiet.org/wiring-matters/years/2025/108-november-2025/rated-current-of-assemblies-for-large-scale-generation/

    Ian Cheek said:

    If it does produce a solution by design, or the “G100” with a correct set of parameters for your installation. There will be cost savings for the disruption of the installation and the associated additional hardware.

    Is it work worth doing, yes. Even if it just proves Ina ≥ In + Igs is the solution, then answer is known. The Design can progress passed all the “what if meetings that tend to stall all good projects”.

    It's not simply whether a G100/2 solution might be OK ... there are  a number of commercial/industrial installations for which the existing requirements of Regulation 551.7.2 are problematic (to say the least).

    Ian Cheek said:
    We wait with breath held for the amendment 4. Does it expand on the notes offered up during the consultation period, or leave it for industry to decide what is meant by, and hoe to implement a Customer Limitation Scheme.

    All anyone could go off, even 'those in the know', is what is in the Draft for Public Comment ... and even that might not fully reflect what is in the version of the standard that is to be published in April 2026.

    I would strongly recommend waiting until 15 April 2026, when this can be fully answered in terms of BS 7671:2018+A4:2026.

  • I would be hesitant to solely apply a risk assessment-style approach. What may be a lightly loaded board now may become oversubscribed as building use changes; coverversely will the generator be turned off during maintenance of downstream plant? For sure an assessment should be carried out to determine a sensible place to connect, but I would advocate that this should be backed up by something more robust, such as a suitable protection scheme or a review of the switchboard design such that overcurrent cannot persist even should that black swan even occur, in order to achieve a departure with an equivalent level of safety to the regs.

    It's worth noting that G100 is only concerned with the power through the point of supply, rather than current through customer-side switchgear. Similar but different, particularly on commercial installations.

    You may well have observed that G100 solutions can, do and could potentially fail to operate correctly, even if left in perfect order at commissioning. Hence the requirement for fail-safe backups for larger installations.

    Ian Cheek said:
    Current supplied from either end of a Bus bar

    Could that mean that some bars to outgoing ways (etc) can draw more than originally intented (e..g. sum of way ratings > incomer setting)? Don't forget that even if it doesn't, this means the switch board will recieve more heat than the original designer will have allowed for.

  • in reply to Jam

    Hi Jam

    These are all points which form part of the review i discussed, it is only in doing the review can you make any forward risk/judgement on a final solution. Each installation will be different, the complication of providing a one solution fits all other than the one currently withing 551.7.2 

    G100 as part of its strategy has a fail safe state, which again as part of any review must form part of any solution. I understand G100 is for DNO protection, unfortunately is is being associated with a Reduction of Solar supply solution regardless of intent.

    The cost of implementing the current 551.7.2 to a site for example with a 1250kVA Transformer with a 1600A ACB onto a switchboard with a Ina of 1600A, becomes financially impractical. One site it was in £Millions/day of loss during the outage required to implement, outweighing any benefit for fitting Solar. The site did not install the Solar, which was a shame at 800kWp.

    Building use does change and any design that does not anticipate this, would be a poor design. While 132.16 seems to leave any amendment or additions within the hands of the next Design, In Solar installations with a 25 year life span, this MUST be part of the Solar designers thought process. EV Charging is going top make a huge impact on current flow and demand within any installation.

    The review should also reflect on the site management of the installation. When looking at site with robust management teams for their infrastructure, control is absolute.

  • in reply to Ian Cheek
    Ian Cheek said:
    The cost of implementing the current 551.7.2 to a site for example with a 1250kVA Transformer with a 1600A ACB onto a switchboard with a Ina of 1600A, becomes financially impractical. One site it was in £Millions/day of loss during the outage required to implement, outweighing any benefit for fitting Solar. The site did not install the Solar, which was a shame at 800kWp.

    Agreed, and this might be where a departure might be considered, with associated suitable design assessment.

    In that specific case one rather hopes that if the cost of an outage on that switchboard was that high a) The installation was designed with some redundancy from generators and/or a split bus which could reduce the impact considerably and b) They have scheduled maintenance outages or fallow periods which are the window for such work (or at least you can add your enabling works to the list of things to do next time one comes up). If neither is the case it's probably not as mission-critical as they might have you believe...

    Thing is that G100 is only a measure of power through a single point, whereas BS7671 551.7.2 is concerned with buses where there are multiple sources of current. With the setup you described, if the CTs were on the switchboard incomer and PV connected through an outgoing way(s), even if set to "zero export" a G100 PPC wouldn't be able to stop the switchboard busbar carrying ~2000A of load (supplied as, for example 900A through the incomer and 1100A from the PV). It may possible for the same equipment to, for example, prevent the total of the incomer current and PV output exceeding the busbar rating, perhaps with some software jiggery pokery and/or additional CTs, but that's not G100 any more.

    It also only fails safe in as far as the comms need to stay alive, unless you are including backup reverse power protection relays in that statement. Inverters and PPCs are frequently connected to the 'net so could be compromised, and it is not unheard of for settings to be reset during, for example, firmware upgrades. For the scale of project suggested, I would suggest additional measures are taken.

  • in reply to Jam

    Hi Jam

    I think you misunderstand when i say people are associating G100 with a Limitation solution. They are incorrectly using it as you have stated its for the control of the DNO side, and not the client side. A bit like when people say Bonding when they mean Earthing.

    G100 has some principles in how it monitors the situation and actions on occurrence and duration. Both these are areas for discussion on the Customer Limitation solution going forward. Its principles rather than its physical interaction.

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