Looking at some terminal blocks like the one below, why are the top level positions color coded blue while the middle ones are in grey? I understand yellow-green are for ground connections. I am guessing Blue is for DC, but can't figure out what grey is usually meant for.
Try this chart for size: -
Phoenix contacts also use the same code except the green/yellow combo is used in the UK (and EU) as ground and GRN in the US): -
The colors are following IEC standards for single phase AC wiring. Green/yellow (green with a yellow stripe) for protective ground, blue for neutral and brown or black (the unmarked grey terminal) for hot. The grey terminal is left unmarked not only due to the ambiguity in the IEC standard, but so the installer may mark it themselves. Individual segments of terminal strips can be ganged together with jumpers or left independent. However, all AC ground and neutral wires are typically connected together (the exception being transformer-isolated circuits). That leaves the hot wire to function as the on/off signal of a control system. In a system with many independently controlled devices, that means many hot wires and a necessity for color labeling beyond simple black or brown.
It depends on the standard that is being followed, but from the panels I have seen, blue is for DC control lines, and the gray is the return.
It looks to me like you have a terminal block set for single phase (IE 120V) power distribution. The Grey would be for 120Vac, Blue for Neutral, and Gn/Ye for earth ground.
Blue is typically reserved for use in Europe, but is accepted here in the US generally.
Hope this helps.
Blue is N (Neutral). That exact component is a Phoenix Contact Ground modular terminal block - UT 2,5-PE/L/N (or similar size). Protective Earth, Line and Neutral. Keep in mind: neutral means something different in EU than it does in USA.
In the EU, cables are often 3 wires: brown, blue and green/yellow. You'd connect blue to blue, GN/YE to GN/YE and brown to grey. Brown terminals are uncommon (unless using Weidmuller, where all terminals are a light beige if uncolored, where Phoenix uses grey).
