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Reading Table 41.1 (U₀ and the three-phase trap)Three-phase: 400 volts or 230 to earth?

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The question at the end: Three-phase: 400 volts or 230 to earth? The answer is in the notes below.

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For circuits that do fall under Table 41.1, you need two assumptions for the UK:

  • The public supply is AC (not DC).
  • The nominal voltage to earth, U0, is 230 V.

Split the table into four voltage columns. Look at the heading symbol U0: it means voltage to earth. On a standard 230 V supply you're in column 2, which is the one you'll use most.

Table 41.1, AC values (TN and TT)
System1 · 50 V < U0 ≤ 120 V2 · 120 V < U0 ≤ 230 V3 · 230 V < U0 ≤ 400 V4 · U0 > 400 V
TN0.8 s0.4 s0.2 s0.1 s
TT0.3 s0.2 s0.07 s0.04 s

The table also gives DC values in each column. You won't need them for a UK AC supply. Check your copy for those.

UK answer: AC, TN, U0 = 230 V → 0.4 s. If it's TT → 0.2 s. So in our example installation, circuits 1 and 4 must disconnect in up to 0.4 s.
L1 L2 L3 N E 400 V line to line 230 V to earth = U₀ Table 41.1 uses U₀ U₀ = 230 V on single-phase AND on 400 V three-phase → column 2 TN 0.4 s · TT 0.2 s
A three-phase circuit is 400 V between phases but still 230 V to earth. Table 41.1 is read by voltage to earth (U0), so it's still column 2.
The three-phase trap
Lots of people see "400 V three-phase" and jump to column 3 or 4. That's not correct. Three-phase circuits are 400 V between phases but still 230 V to earth, so you still use column 2. Only move columns if the question actually says the voltage to earth is something else.
About marking your book
You may see the four columns marked 1 to 4 in someone else's book. Don't write in yours. You can't take a marked-up book into the exam.

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