Toshiba GRZ100 B Series, Instruction Manual, Page: 70 / 447

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6 F 2 S 0 8 4 6
Setting
The setting elements necessary for the definite time overcurrent backup protection and their
setting ranges are shown below.
Element Range Step Default Remarks
OC 0.5 - 100.0 A 0.1 A 6.0 A Phase overcurrent
( 0.1 - 20.0 A 0.1 A 1.2 A) (*)
TOC 0.00 - 10.00 s 0.01 s 3.00 s OC delayed tripping
EF 0.5 - 5.0 A 0.1 A 1.0 A Residual overcurrent
( 0.10 - 1.00 A 0.01 A 0.20 A) (*)
TEF 0.00 - 10.00 s 0.01 s 3.00 s EF delayed tripping
[OCBT] OFF/ON ON OC backup protection
[EFBT] OFF/ON ON EF backup protection
[EFBTAL] OFF/ON ON EF backup trip alarm
(*) Current values shown in the parentheses are in the case of 1 A rating. Other current values are in
the case of 5 A rating.
2.4.6 Thermal Overload Protection
Thermal overload protection is provided with GRZ100 model 100, 200, 300 series.
The temperature of electrical plant rises according to an I
2
t function and the thermal overload
protection in GRZ100 provides a good protection against damage caused by sustained
overloading. The protection simulates the changing thermal state in the plant using a thermal
model.
The thermal state of the electrical system can be shown by equation (1).
θ =
I
I
e
AOL
t
2
2
1 100 −
⎛
⎝⎜
⎞
⎠⎟
×
−
τ
% (1)
where:
θ
= thermal state of the system as a percentage of allowable thermal capacity,
I = applied load current,
I
AOL
= allowable overload current of the system,
τ
= thermal time constant of the system.
The thermal state 0% represents the cold state and 100% represents the thermal limit, which is the
point at which no further temperature rise can be safely tolerated and the system should be
disconnected. The thermal limit for any given system is fixed by the thermal setting I
AOL
. The relay
gives a trip output when
θ
= 100%.
The thermal overload protection measures the largest of the three phase currents and operates
according to the characteristics defined in IEC60255-8. (Refer to Appendix O for the
implementation of the thermal model for IEC60255-8.)
Time to trip depends not only on the level of overload, but also on the level of load current prior to
the overload - that is, on whether the overload was applied from ‘cold’ or from ‘hot’.
Independent thresholds for trip and alarm are available.
The characteristic of the thermal overload element is defined by equation (2) and equation (3) for
‘cold’ and ‘hot’. The cold curve is a special case of the hot curve where prior load current Ip is
zero, catering to the situation where a cold system is switched on to an immediate overload.