diff --git a/slsDetectorServers/ctbDetectorServer/RegisterDefs.h b/slsDetectorServers/ctbDetectorServer/RegisterDefs.h
index bf3dae964..8aa6c2ff7 100755
--- a/slsDetectorServers/ctbDetectorServer/RegisterDefs.h
+++ b/slsDetectorServers/ctbDetectorServer/RegisterDefs.h
@@ -439,8 +439,8 @@
 
 #define POWER_ENBL_VLTG_RGLTR_OFST          (16)
 #define POWER_ENBL_VLTG_RGLTR_MSK           (0x0000001F << POWER_ENBL_VLTG_RGLTR_OFST)
-#define POWER_HV_SLCT_OFST                  (31)
-#define POWER_HV_SLCT_MSK                   (0x00000001 << POWER_HV_SLCT_OFST)
+#define POWER_HV_INTERNAL_SLCT_OFST         (31)
+#define POWER_HV_INTERNAL_SLCT_MSK          (0x00000001 << POWER_HV_INTERNAL_SLCT_OFST)
 
 /* Number of Words RW register TODO */
 #define NUMBER_OF_WORDS_REG                 (0x5F << MEM_MAP_SHIFT)
diff --git a/slsDetectorServers/ctbDetectorServer/slsDetectorFunctionList.c b/slsDetectorServers/ctbDetectorServer/slsDetectorFunctionList.c
index df2281e81..b6b7e482c 100755
--- a/slsDetectorServers/ctbDetectorServer/slsDetectorFunctionList.c
+++ b/slsDetectorServers/ctbDetectorServer/slsDetectorFunctionList.c
@@ -1422,14 +1422,14 @@ int setHighVoltage(int val){
 	    FILE_LOG(logINFO, ("Setting High voltage: %d V\n", val));
 		uint32_t addr = POWER_REG;
 
-		// switch off high voltage
-		bus_w(addr, bus_r(addr) & (~POWER_HV_SLCT_MSK));
+		// switch to external high voltage
+		bus_w(addr, bus_r(addr) & (~POWER_HV_INTERNAL_SLCT_OFST));
 
 		MAX1932_Set(val);
 
-		// switch on high voltage if val > 0
+		// switch on internal high voltage, if set
 		if (val > 0)
-		    bus_w(addr, bus_r(addr) | POWER_HV_SLCT_MSK);
+		    bus_w(addr, bus_r(addr) | POWER_HV_INTERNAL_SLCT_OFST);
 
 		highvoltage = val;
 	}