Learnings

Outcomes

Test rigs were constructed to test: domestic type cut-outs in various mock enclosures; streetlight type cut-outs in a mock galvanised steel streetlight; singles/looped service-cable specimens in mock domestic enclosures with internal (wall cavity) and externally mounted hockey sticks. For comparison a clipped-direct service cable installation test rig was constructed to test a single service-cable specimen. Solar gain was simulated on the rigs using a simulated solar radiation source with an intensity of 600W/m2.
Limit temperatures were taken from BS7657:2022: 90°C for Cross linked polyethylene (XLPE)and 70°C for poly vinyl chloride(PVC). PVC is used for the outer sheath of the XLPE cables.
The extent to which the different enclosure types affect the amount of de-rating required to prevent the permitted temperatures has been assessed:
1) In the case of domestic cut-outs:
a. The de-rating required is higher for the higher current rated cut-outs (100A) than the lower rated cut-outs (60A/80A).
b. An outdoor Glass reinforced plastic (GRP) meter cabinet enclosure, subject to solar radiation, required the greatest de-rating. Outdoors (non-solar) required the least demanding de-rating and the indoor enclosure lies between the two.
2) In the case of streetlight cut-outs:
a. The analysis of all 4mm2 Separate Neutral & Earth (SNE) temperature data showed that none of the temperatures exceeded the BS7870-3.21:2011 maximum permissible temperature of 70°C, therefore, there was no requirement for de-rating.
b. None of the cut-outs required de-rating when the enclosure was not subjected to simulated solar radiation.
c. Most of cut-out types did not require de-rating even when the enclosure was subjected to simulated solar radiation. The two types of cut-out in table 13 in the final report ‘ENA CEP026 NET ZERO Termination Project - Final Report 2023’ that may warrant de-rating may require further investigation due to possible rogue behaviour of individual cutouts distorting the results. These are the Henley Type 5 and the Lucy Type 2.

Table 13

d. In the case of the service-cables: (note that the ranges stated are the differences between the derating factors found for tests on combinations of solar/non solar, Combined Neutral & Earth (CNE)/SNE, external and internal hockey stick (HS) tube, and 25/35mm2 cables)
e. The cross-section of the copper neutral conductors was less than expected.
f. The most severe de-rating required was to 33%, of nominal (35A c.f. 105A), that is reduced by 67%. This was for 25mm2 CNE cable, single (not looped) service, external hockey stick with solar, based on the maximum temperature measured of the triple sets rather than the average. Based on average temperature {A}, the factor is 35%.
g. The de-rated (to) factors, average (and range) were: 25mm2 non-solar: 72% (66 to 78%); 35mm2 non-solar: 73% (66 to 81%); 25mm2 solar: 50% (35 to 64%); 35mm2 solar: 54% (35 to 65%). These are based on the temperature meas-urements averaged over the triple sets of thermocouples.
h. The single cable external conditions would be closest to the specified rating for cable in-conduit, clipped to wall; the calculated decreased ratings were down to around 72% to 79% of the specified level. This may be partly due to the extended loading time in these tests.
i. SNE cable performed better than CNE Cable by -1 to 11%.
j. External Hockey Stick (HS) cable performed better than internal (cavity) hockey stick by 2 to 12 % without solar; Exter-nal HS performance was reduced compared to internal by 8 to 24% with solar irradiation.
k. Internal HS cable capacity was reduced by 7 to 10% by solar irradiation. External HS cable capacity was reduced by 22 to 37% by solar irradiation..
l. 35mm2 looped cables would not fit in the standard 39mm ID hockey stick, so a 50mm waste pipe was used. Two 25mm2 cables were a tight fit in the 39mm ID hockey stick, which may have produced better thermal conduction for the cable-generated heat. The other cables were not tight.
3) Solar gain was the factor that caused greatest derating of outdoor enclosures, to avoid cut-out component part temperatures exceeding the maximum permissible temperatures defined in BS7657:2022.
a. The solar gain of the meter enclosures is heavily dependent upon the colour of the enclosure. Different de-rating fac-tors may be appropriate depending upon enclosure colour.
b. The degree of solar radiation depends on various factors, including latitude, weather, cloud cover, time of year, time of day, and the direction that the enclosure faces. Changes in ambient temperature and wind speed may also affect the temperature rise that takes place.
4) Solar radiation generally produced an increase in cut-out component parts of around 20- 25℃ in white outdoor GRP meter cabinets, and around 10-12℃ in streetlight enclosures, in addition to the heating caused by the electrical load.
5) The times to thermal stability (less than 1℃ change per hour) of the energised cut-outs in the various enclosures have been assessed. For domestic cut-out enclosures the time taken is typically 2 hours in free air; 4.5 hours in the indoor wooden cup-board, and 3 hours in the outdoor GRP meter cabinet. In the case of a streetlight cut-out mounted in a streetlight compart-ment, the time is around 1.5 hours. Considering LCT loads, these times are all within the plausible duration of an EV charge cycle. Cut-outs may reach the temperatures observed during the testing due to load current electrical heating during EV charg-ing.
6) The solar gain heating thermal time constants have been assessed for each of the outdoor enclosure types. Solar heating will depend on factors including sunshine duration, the sun’s azimuth relative horizontal position and elevation through the day, as well as any wind cooling.
a. In the case of the streetlight, the time taken for the cut-out component parts to reach their final temperature as a result of solar gain is around 4 hours. A streetlight exposed to direct sunshine for more than a few hours is likely to reach maximum temperatures produced by solar gain. As streetlights will typically be tubular in shape, changes in the sun’s angle of azimuth during the day will have minimal effect on the level of radiation received, so it is not unreason-able to expect extended periods of solar gain.
b. The solar heating thermal time constant for the white outdoor GRP meter cabinet enclosure is significantly longer at around 10 hours, so real conditions may mitigate the derating factors found. This is important because outdoor enclo-sures exposed to solar radiation are potentially the worst-case scenario for domestic cut-out derating with LCT loads. Further work, including outdoor measurements in natural sunlight, would be required to quantify the degree of miti-gation that might be expected. Limiting EV charging duration at certain times of the day may be another mitigating approach that could be adopted.

7) Tariff meters and switched-isolators in the domestic meter enclosures dissipated significant electrical power and may overheat themselves. In some instances, the meter and isolator dissipated more power and get hotter than the cut-out. The power dissi-pated by a switched-isolator may vary unpredictably with time: this may be due to fluctuations in contact resistance.

Lessons Learnt

The significant levels of calculated de-rating depend on the use of constant 600W/m2 as the level of insolation. This was determined via the stakeholder engagement sessions held during the project. Further work is recommended to assess the validity of this approach and the results:
1) Theoretical study to better model daily radiation levels and temporal distribution.

a. Outdoor testing with temperature monitoring and radiative flux measurements.

b. Site tests to measure temperature rise times, possibly utilising ‘MyElectricAvenue’ facilities.

c. Wider site survey using temperature indicator strips to verify scale of issues found.

d. Review original approval testing of meter enclosures for comparison with the test and results in this report.

e. Depending upon the results of the previous points, it may be appropriate to perform laboratory tests to compare the constant irradiation implemented with an improved model.

f. Depending upon the results of the previous points, revise the de-ratings found in this report.

2) Further testing to cover a wider range of equipment may be considered:
a. Check effects of outdoor meter colour enclosure (and possibility of shielding or reflective coating).

b. Investigate wider range of domestic and streetlight cut-outs.

c. Investigate the effects of and on tariff meters and switched-isolators that generated heat and were excluded from tests.

d. Comparative testing of other types of service cables including 4mm2 CNE.

e. Streetlight looped service tests.

f. Other ex-service cut-outs, as those tested performed less well than the new cut-outs., these ex-service cutouts were used due to the fact they are no longer in production.

g. Investigate possibility of overheating of PVC meter tail at cut-out.

3) Investigate effect of high short loads such as showers combined with the LCT effect.

4) Further analysis of A2803/CEP025 report results to identify and resolve anomalies and out-of-scope issues arising.

5) Determine if any smart tariff meters measure their own temperature, possibly leading to survey of actual installations; or ongoing monitoring of temperatures; or inclusion in future requirement specifications.

6) Address the points raised in Conclusion C12 of Final report ‘ENA CEP026 NET ZERO Termination Project - Final Report 2023’