Littelfuse Industrial Shock Block SB5000 Series, Manual, Page: 16 / 21

SB5000 Industrial Shock Block
Page 12
Rev. 1-B-101821
4
.
UL 943C R
EQUIREMENTS
The following sections contain excerpts from the
UL 943C
(1)
Standard for Class C and D applications. For
EGFPD models, only Section 4.1 applies where selectable
trip levels from 6 to 100 mA are permitted
(20 mA is the only allowed trip level for GFCI devices in
NEC applications).
4.1 T
RIP
T
IME
Class C and Class D GFCIs shall be capable of
interrupting the electric circuit to the load when the fault
current to ground (
I ) is within the range of a minimum of
20 mA through a maximum of (110% of the rated
voltage/500 Ω)
A within the time interval ( T ) in
accordance with the relationship:
Where:
T is expressed in seconds, and
I is expressed in mA
Except that T is not required to be less than 20 ms.
The tripping threshold of these GFCIs shall be in the
range of 15 to 20 mA.
Fig. 14 shows the Class-A GFCI maximum-trip-time-
versus-current curve defined by UL 943 (with the above
formula); UL 943 also defines a limit to the formula, with
a minimum required trip time (or fastest trip speed) of
20 ms, which occurs at about 300 mA of ground-fault
current (
I ) per the formula. Class C and D GFCIs must
operate within this same curve, except that their operating
threshold is 15 to 20 mA (non-adjustable). See Fig. 15.
The SB5000 EFGPDs described in this manual also
operate within the Class-A formula, but have a 6- to
100-mA threshold setting range.
4.2 G
ROUNDING
C
IRCUIT
The equipment grounding circuit of the circuit shall be
sufficiently low impedance that, should a fault to ground
occur, the voltage resulting across the grounding circuit
shall not exceed 150 V. The size of the grounding
conductor required for a circuit shall be determined by
calculating the resistance using the following formula and
converting that value to a wire gauge based on the length
of the equipment grounding conductor:
Where:
R
G
is the resistance of the equipment grounding
conductor
R
U
is the resistance of the ungrounded conductor, and
V
LG
is the RMS value of the supply line to ground
voltage
Refer to UL 943C Appendix A for a sample calculation
of grounding conductor size.
4.3 G
ROUNDING
M
ONITOR
/I
NTERRUPTER
A GFCI shall be capable of detecting and causing
interruption of the circuit under both of the following
conditions:
a) An open grounding circuit, and
b) Impedance in the grounding circuit that would allow
more than a 150 volt drop in the grounding circuit.
(1)
Excerpts from UL 943C are the property of
Underwriters Laboratories Inc.
FIGURE 12. Class-A GFCI Curve.
FIGURE 13. Class-C and -D GFCI Curve.
T
=
1.43
20
I
( )
10
0.1
20 ms
0.01
0 6
100 200 300
TIME (s)
CURRENT (mA)
1
10
0.1
20 ms
0.01
0 20 100
200 300
TIME (s)
CURRENT (mA)
1