FAQ
TL;DR: For fixed connector switchboards, a disconnection time of E5 s can be acceptable; "It all depends on the classification of the device." Choose the K factor accordingly, then verify Zs against your measured loop. [Elektroda, Miniax, post #17670441]
Why it matters: This FAQ helps site electricians and designers choose K and check Zs so WT gG/gL 80A fuses meet disconnection times.
Quick Facts
- 80A belongs to WT type 2; at 0.2 s use K = 11.1 for calculations. [Elektroda, opornik7, post #17668690]
- Measured Zs = 0.29Ω; with K = 11.8 at 0.2 s, Zs_max ≈ 0.24Ω, so it failed quick disconnection. [Elektroda, anet870, post #17668644]
- Permanent, protected connector boards may assume E5 s disconnection time; device classification governs selection. [Elektroda, Miniax, post #17670441]
- In this project, the designer accepted K = 5.4 for a 5 s basis of design. [Elektroda, anet870, post #17680317]
- 4×35 mm² YAKY feeders use PEN upstream; evaluate the L–PEN loop, not L–N, before the split. [Elektroda, kkas12, post #17681342]
What K coefficient should I use for an 80A gG/gL fuse at 0.2 s?
Use K = 11.1 for an 80A insert at 0.2 s. 80A belongs to the WT type 2 group, not type 3. "For 80A the k factor is 11.1 at a time of 0.2s." [Elektroda, opornik7, post #17668690]
When is a 5 s disconnection time acceptable for socket boards?
Use E5 s for a permanent connector switchboard with its own protective devices and normal conditions. Classification drives the choice. "It all depends on the classification of the device." Document the device class and environment. [Elektroda, Miniax, post #17670441]
For 5 s, what K value did the designer accept for the 80A case here?
The designer agreed to use K = 5.4 for a 5 s disconnection time. They referenced the same rationale about classification and permanence. [Elektroda, anet870, post #17680317]
My measured Zs is 0.29Ω; does that meet the 0.2 s requirement?
In this thread, no. With K = 11.8 at 0.2 s, Zs_max was 0.24Ω for 80A at 230 V. The measured 0.29Ω exceeded the limit, so quick shutdown was not met. [Elektroda, anet870, post #17668644]
How do I calculate the maximum permissible Zs using K and In?
Follow this three-step check:
- Select the target disconnection time and K from the fuse table.
- Compute Zs_max = 230 V F (K × In).
- Compare with your measured Zs; if Zs > Zs_max, adjust design.
Example from the thread: 230 F (11.8 × 80) ≈ 0.24Ω. [Elektroda, anet870, post #17668644]
On a 4×35 mm² YAKY feeder without a separate PE, which loop do I evaluate?
Treat the upstream conductor as PEN until the split to PE and N. Calculate and test the L–PEN loop, not L–N. "So not L1-N but L1-PEN." [Elektroda, kkas12, post #17681342]
For Zs checks, should I use 230 V or 400 V in calculations?
The thread compared both. Using 230 V gave Zs_max ≈ 0.53Ω, while 400 V gave ≈ 0.92Ω. These figures came from the same installation example. Confirm the applicable fault path before choosing the voltage. [Elektroda, anet870, post #17680317]
Is an 80A WT insert “type 3” or “type 2”?
A contributor advised that 80A belongs in type 2, not type 3. They also provided the K value for 0.2 s at 80A. This affects which table row you use. [Elektroda, opornik7, post #17668690]
If quick disconnection at 0.2 s is not met, what should I change?
One recommendation was to redesign the cable or reduce the fuse rating. "Either modernize the cable or go down to 63A." This action avoids non-compliant Zs. [Elektroda, Strumien swiadomosci swia, #17668748]
Does the connector’s class (I or II) change the disconnection time choice?
Yes, classification influences time selection. A fixed board with its own protections can use E5 s if conditions match. "It all depends on the classification of the device." Document the class in your report. [Elektroda, Miniax, post #17670441]
Where can I find K coefficients for WT gG/gL inserts?
Use the manufacturer’s fuse tables. In this thread, the ETI table informed K for time-based checks. The OP cited K = 11.8 for 0.2 s as an example. [Elektroda, anet870, post #17668644]
Would increasing cable cross-section help meet Zs limits?
A participant suggested upgrading the conductor if the design fails the Zs check. "If it doesn’t work, get one with a larger cross-section." This lowers loop resistance and improves disconnection. [Elektroda, Strumien swiadomosci swia, #17668773]
