Re: TVSS Sizing


Transient surges (SEMP) from distance  do not have a high energy. Max to calculate In = 20kA, wave form 8/20µs, Imax = 40kA 8/20µs (1 time). You can use a Varister SPD like LEUTRON EP C TNS -350 for that which will not trip the fuse

But if you have a building with an external lightning protection and lightning stroke is entering the franklin rod (air terminal) a very high energy flow to ground (acc. to Lightning protection standard IEC 62305 part 1 to 4 : LPL 1 max. 100kA 10/350µs….and not 8/20µs like very old standard says).

50% of this value will enter the electric system in the building via grounding points and Equipotential bonding bar (EBB) and will destroy TVSS SPD (Surge Protective Device) as well as equipment.

So you should install in the first main distribution board at service entrance a lightning protective SPD with 25kA 10/350µs per line for L1, L2, L2, N and connect to EBB in the mainboard. Max length of wire between L and PE at EBB total 1m. Cross sectional area between PE of SPD and ground (EBB): 25mm² or higher.

You should use for that a 4-pole GDT spark gap SPD like LEUTRON PP BCD TNS 25/100 with remote monitoring sensor /FM with not resettable N.C. contact.

Directly at the equipment to be protected an voltage limiting SPD of test class III has to be installed max distance to grounded equipment 2m. Link: