Double tap transformer?

pc1966 said:

Do they do something similar with the MV/HV side? I.e. have separate feeds to each transformer, but fed via switches and a crossover-switch so you can have one HV feed used for both transformers in the event some part of the HV grid is out?

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Yes, sort of.

For major sites, the intake is usually from two different MV/HV independent routes.

So even if one grid connection goes there is still supply via the other route.

In this case the intakes are usually spread over the (large) site, and there are interconnections acting like the one board.

On larger sites there may be multiple intakes to the same MV switchboard set up similar to my earlier post, with interconnections as above.

I have worked on many sites that take in at 33kV and above and have an 11kV distribution set up as complex as many utilities (city based), which often includes 11kV motors, generators and so on.

Unfortunately they vary in quality from modern and well maintained, to something scaringly different!!!

Julie. said:

Depends on the arrangements, but there are a number of ways.

For simple applications you could bring the secondary onto the LV busbar, which would then have multiple feeders out, each rated for their corresponding loads , could be fuse switches, mccb, acb or whatever.
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We would pretty much never have a long feed from the transformer to the disconnector - you would split in the substation to then go to all buildings, or you would build the sub in the one building and feed it and the others from there.

Very minimal unprotected length of secondary cables.

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Ahh see, this is what I am interested in. What are the BST rules on the length of the unprotected length of secondary cable?

JWallstrop said:

Ahh see, this is what I am interested in. What are the BST rules on the length of the unprotected length of secondary cable?

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None.

I don't think you quite understand, our rules are not like your rules, (but a bit different,) they follow a quite different philosophy.

You have it spelt out "x length in these circumstances" etc etc.

Our rules are "distribution must disconnect within 5 seconds " etc etc.

Therefore there isn't actually any unprotected cable (although I and others use the term), it really means there isn't "our" protection on the cable.

In the case of a transformer the cable from the secondary to the switchgear bus, and the secondary itself is protected by the primary side protection.

How long therefore - well it must disconnect in 5s in the event of a fault.

So ulike your codes, the length isn't specified, it is determined by the characteristics of the installation.

(Utilities don't necessarily have this rule, although they usually follow the 5s just the same now. They used to have whatever was the limiting time, so if the transformer can withstand a direct short for 15s, but adiabatic calculations on the secondary cable indicate 14s, but then again adiabatic on the primary cable is 10s, then the design limit would be less than 10s)

The only time length is used is where it is actually arbitrary because the installer doesn't have the information.

So for example, a utility will provide a supply to a customer, it will have a fuse (100A, 80A, 60A or rarely 40A) - which size you probably don't know.

The installer then knows that it must go through their own protective device within 3m - could be a simple fuse switch, or could be the actual distribution panel (known as the consumer unit - cu here) with fuses/mcb/rcbo...

This philosophy follows right through our regulations, final circuits don't have a maximum length defined , that comes out of calculations, it must disconnect within x seconds (depends on the type of installation/earthing arrangements), and have less than y voltage drop... etc.

There are guide books that rationalise the above into very conservative guidelines, so on a typical installation, on a 6A type B mcb using 1.5mm^2 cable "z" metres of cable is the maximum length to stay within volt drop and disconnect times etc.

These are not part of the regulations, just shortcuts reducing the need for the calculations.

Just to add - if it's a customer owned transformer, then the utility will provide the information of the MV side, so something like "11kV 107MVA , supplied via a fuse switch having 63A fuses type 12TDLEJ63 or equivalent"

Then you have all the information to design the installation yourself, you just select an appropriate transformer - giving you the % voltage impedance, an appropriate set of cables and switchboard etc and of course the maximum cable length drops out via your calculations, - not suitable? Well start again, relocate transformer or switchboard or change cable size etc.

As @Julie. says our regulations are much more calculation-based in terms of what is allowed. In terms of the LV side then most circuits need overload protection (regulation 433.2), unless fixed loads where that is not likely and some other details (regulation 433.3), but practically all circuits need fault protection (regulation 434).

The closest we have to your rules is the 3m limit on conductors not adequately protected in themselves (but protected down-stream) which typically is taps of a bus-bar chamber and the case mentioned above where the DNO supply is not known to protect the first stage of the installation.

Here is the main bit for 433 on overload:


And here is the main bit for 434 fault protection:




Apologies for the warped photos of the regs book!

pc1966 said:

As @Julie. says our regulations are much more calculation-based in terms of what is allowed. In terms of the LV side then most circuits need overload protection (regulation 433.2), unless fixed loads where that is not likely and some other details (regulation 433.3), but practically all circuits need fault protection (regulation 434).

The closest we have to your rules is the 3m limit on conductors not adequately protected in themselves (but protected down-stream) which typically is taps of a bus-bar chamber and the case mentioned above where the DNO supply is not known to protect the first stage of the installation.

Here is the main bit for 433 on overload:
View attachment 93813
View attachment 93814
And here is the main bit for 434 fault protection:
View attachment 93815

View attachment 93816

Apologies for the warped photos of the regs book!

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But the warped photos, appear so appropriate........