RB Low and MB Low meanings
On the SIMATIC S5 system described, the two indicators monitor separate backup supplies. Replacing a lithium or external battery will not clear RB low because that indicator refers to the rechargeable battery circuit.
| Indicator | Monitored supply | Warning condition |
|---|---|---|
| RB low | Rechargeable battery | Voltage below the preset 3 V limit |
| MB low | Lithium battery or external battery | Voltage below the preset 3 V limit |
Why RB Low remains on after battery replacement
The reported system uses CPU 6ES5928-3UB21 and central rack 6ES5188-3UA12, but the power-supply model is not identified. First determine which battery was replaced. If it was the lithium or external battery, it addressed the MB circuit rather than the RB circuit.
The available evidence also reports that the rechargeable battery block may be delivered disconnected and is located in the third ventilator. Treat that location as configuration-dependent until the installed power-supply documentation confirms it. A disconnected rechargeable battery, a failed battery block, or an RB-circuit voltage below 3 V remains consistent with the illuminated RB indicator.
Diagnostic procedure
- Record whether RB low, MB low, or both indicators are illuminated.
- Identify the installed power-supply model and confirm the battery locations against its documentation before disconnecting anything.
- If RB low is illuminated, inspect the rechargeable battery block and its connector. Check the third-ventilator location if it applies to the installed assembly.
- Measure the rechargeable battery voltage using the documented test method. If it is below the 3 V limit, replace the rechargeable battery rather than the lithium or external battery.
- Check whether the reset switch has been left in the reset position. The evidence identifies this as a diagnostic question but does not establish the required switch sequence, so use the power-supply manual before operating it.
- Verify that RB low clears after restoring a connected rechargeable battery circuit above the stated limit. If it remains illuminated, investigate the connector, wiring, and power-supply monitoring circuit rather than replacing the external battery again.
FAQ
What does RB low mean on a Siemens S5 power supply?
RB low means the rechargeable battery voltage is below the preset 3 V limit. It does not refer to the lithium or external battery.
What does MB low mean on a Siemens S5 system?
MB low means the lithium battery or external battery voltage is below the preset 3 V limit. Replacing that battery addresses MB low, not RB low.
Why is RB low still on after installing a new battery?
The replacement may have been the MB lithium or external battery instead of the RB rechargeable battery. Inspect the rechargeable battery block, its connection, and its voltage relative to the 3 V limit.