Air carrier flight crew reported descending prior to crossing restriction on approach.

Attributes
ACN	1618329
Time
Date	201902
Local Time Of Day	0001-0600
Place
Locale Reference	ORD.Airport
State Reference	IL
Environment
Flight Conditions	IMC
Aircraft 1
Make Model Name	Widebody Transport
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Flight Plan	IFR
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 178 Flight Crew Total 11012 Flight Crew Type 1910
Person 2
Function	First Officer Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 160 Flight Crew Total 16061 Flight Crew Type 7834
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance Inflight Event / Encounter Weather / Turbulence

Narrative:

On approach 28C approximately 20 NM final at 5;000 MSL; we were cleared to maintain 5;000 MSL until established clear for the ILS 28C. First officer (first officer) requested 4;000 feet; captain set 4;000 feet while talking on radio and identifying the ILS. When I checked the step down altitudes; I realized wrong altitude was set but descend was already initiated. The correct step down was to stay at 5;000 feet until rzuko; then step down to 4;000 MSL to hazil; we arrived rzuko at 4;000 MSL. All sops were followed; however an error was made. We stayed at 4;000 MSL until glideslope capture; followed by normal approach and landing. There were no traffic conflicts throughout the approach.

Original NASA ASRS Text

Title: Air Carrier flight crew reported descending prior to crossing restriction on approach.

Narrative: On approach 28C approximately 20 NM final at 5;000 MSL; we were cleared to maintain 5;000 MSL until established clear for the ILS 28C. FO (First Officer) requested 4;000 feet; Captain set 4;000 feet while talking on radio and identifying the ILS. When I checked the step down altitudes; I realized wrong altitude was set but descend was already initiated. The correct step down was to stay at 5;000 feet until RZUKO; then step down to 4;000 MSL to HAZIL; we arrived RZUKO at 4;000 MSL. All SOPs were followed; however an error was made. We stayed at 4;000 MSL until glideslope capture; followed by normal approach and landing. There were no traffic conflicts throughout the approach.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.