CT1 and CT2

Also I think Hager were originally French - so are well used to TT installations (the norm in France) - so would expect their product to be TT compatible (i.e CT2).

   - Andy.

TT is not exactly unknown in the UK, you just need to go outside the towns and cities. There are plenty of farms with TT 3 phase supplies from meandering 11kV overheads with a transformer on an H-pole in or just outside the boundary - and those are exactly the sort of places where a CT2 surge arrest makes sense (along with a multiplicity of time delay 100ma or 300mA RCDs ... and a little fence or walled off region around the earth electrodes to keep the animals off it.)
M.

The reason I ask about the series arrangement is that the overvoltage withstand requirement, Uw, for equipment as set out in Table 443.2 is for live conductors to earth. In a CT2 connection could the clamp voltage to earth be twice the individual SPD clamp voltage or is the connection to neutral an effective voltage divider?

So say an overvoltage is in the order of 2KV line to earth. A CT2 method is employed with a 1.5KV SPD between line and neutral and a 1.5KV SPD neutral to earth. Given the earthed neutral, will the line to neutral SPD short? 

The reason I ask about the series arrangement is that the overvoltage withstand requirement, Uw, for equipment as set out in Table 443.2 is for live conductors to earth. In a CT2 connection could the clamp voltage to earth be twice the individual SPD clamp voltage or is the connection to neutral an effective voltage divider?

So say an overvoltage is in the order of 2KV line to earth. A CT2 method is employed with a 1.5KV SPD between line and neutral and a 1.5KV SPD neutral to earth. Given the earthed neutral, will the line to neutral SPD short? 

In the example you give, you are correct.

Consider as the pulse comes along, the leading edge of it causes the L voltage to rise, but in this case for whatever reason not the N.  Once  voltage between L-N is > than the breakdown of the first clamp, current will flow and the L-N voltage will start to be be limited, and as LE voltage limits, the NE voltage will start to rise. Note that even on a TNC supply, unless the surge arrestor  is very close to the N-E bond, the neutral voltage will also start to rise relative to earth once the LN device has broken down and started to pass current.

An observer at the SPD cannot see the short circuit, distance 'd' away, until a time 2* d/c later - this is the time it takes the current to get down the wires from the SPD, to the NE bond and back again - this delay is measured in nano-seconds per foot so does not affect normal 50Hz thinking, which is quite pedestrian, but is important for fast edged pulses  -it is a short circuit at a distance....

The NE device may never break down if the NE bond is near enough to take effect before the voltage on the second arrestor has risen to the breakdown voltage.

M

If this is not helping come back and I can try another tack.

Bear in mind that such a device may well start conducting at around a 500V difference, but due to inductive effects, lead lengths etc, it may only clamp the effective voltage between the two lines to 1.5kV. So a 2kV difference across 2 SPDs in series may well cause them both to start conducting. But I defer completely to Mike over the details.

also true, I was trying to gloss over the rather non-linear turn on curve.

M.