diff --git a/secop_psi/ips_mercury.py b/secop_psi/ips_mercury.py
index c3ee708..252f755 100644
--- a/secop_psi/ips_mercury.py
+++ b/secop_psi/ips_mercury.py
@@ -87,7 +87,6 @@ class Field(MercuryChannel, Magfield):
         return self.query('PSU:SIG:SWHT', off_on)
 
     def write_switch_heater(self, value):
-        super().write_switch_heater(value)
         return self.change('PSU:SIG:SWHT', value, off_on)
 
     def read_atob(self):
diff --git a/secop_psi/magfield.py b/secop_psi/magfield.py
index 65a33af..3efa025 100644
--- a/secop_psi/magfield.py
+++ b/secop_psi/magfield.py
@@ -80,17 +80,19 @@ class Magfield(HasLimits, Drivable):
         'wait time to ensure current is stable', FloatRange(0, unit='s'), readonly=False, default=6)
     wait_stable_field = Parameter(
         'wait time to ensure field is stable', FloatRange(0, unit='s'), readonly=False, default=31)
+    persistent_limit = Parameter(
+        'above this limit, lead currents are not driven to 0',
+        FloatRange(0, unit='$'), readonly=False, default=99)
 
     _state = None
     __init = True
-    _super_sw_check = False
     _last_target = None
-    switch_time = None
+    switch_on_time = None
+    switch_off_time = None
 
     def doPoll(self):
         if self.__init:
             self.__init = False
-            self.switch_time = time.time()
             if self.read_switch_heater() and self.mode == Mode.PERSISTENT:
                 self.read_value()  # check for persistent field mismatch
                 # switch off heater from previous live or manual intervention
@@ -112,6 +114,7 @@ class Magfield(HasLimits, Drivable):
 
     def initModule(self):
         super().initModule()
+        self.registerCallbacks(self)  # for update_switch_heater
         self._state = StateMachine(logger=self.log, threaded=False, cleanup=self.cleanup_state)
 
     def write_target(self, target):
@@ -180,12 +183,16 @@ class Magfield(HasLimits, Drivable):
             return Retry()
         return self.start_switch_on
 
-    def write_switch_heater(self, value):
-        """implementations must super call this!"""
-        self._super_sw_check = True
-        if value != self.switch_heater:
-            self.switch_time = time.time()
-        return value
+    def update_switch_heater(self, value):
+        """is called whenever switch heater was changed"""
+        if value:
+            self.switch_off_time = None
+            if self.switch_on_time is None:
+                self.switch_on_time = time.time()
+        else:
+            self.switch_on_time = None
+            if self.switch_off_time is None:
+                self.switch_off_time = time.time()
 
     def start_switch_on(self, state):
         """switch heater on"""
@@ -193,10 +200,7 @@ class Magfield(HasLimits, Drivable):
             self.status = Status.PREPARING, 'wait for heater on'
         else:
             self.status = Status.PREPARING, 'turn switch heater on'
-        self._super_sw_check = False
         self.write_switch_heater(True)
-        if not self._super_sw_check:
-            raise ProgrammingError('missing super call in write_switch_heater')
         return self.switch_on
 
     def switch_on(self, state):
@@ -204,7 +208,11 @@ class Magfield(HasLimits, Drivable):
         if (self.target == self._last_target and
                 abs(self.target - self.persistent_field) <= self.tolerance):  # short cut
             return self.check_switch_off
-        if state.now - self.switch_time < self.wait_switch_on:
+        self.read_switch_heater()
+        if self.switch_on_time is None:
+            self.log.warning('switch turned off manually - try again')
+            return self.start_switch_on
+        if state.now - self.switch_on_time < self.wait_switch_on:
             return Retry()
         self._last_target = self.target
         return self.start_ramp_to_target
@@ -255,7 +263,6 @@ class Magfield(HasLimits, Drivable):
         else:
             self.status = Status.FINALIZING, 'wait for heater off'
         self.write_switch_heater(False)
-        # no check for super call needed here (would have been detected in start_switch_on)
         return self.switch_off
 
     def switch_off(self, state):
@@ -265,8 +272,15 @@ class Magfield(HasLimits, Drivable):
             self._last_target = None
             return self.start_switch_on
         self.persistent_field = self.value
-        if state.now - self.switch_time < self.wait_switch_off:
+        self.read_switch_heater()
+        if self.switch_off_time is None:
+            self.log.warning('switch turned on manually - try again')
+            return self.start_switch_off
+        if state.now - self.switch_off_time < self.wait_switch_off:
             return Retry()
+        if abs(self.value) > self.persistent_limit:
+            self.status = Status.IDLE, 'leads current at field, switch off'
+            return self.finish_state
         return self.start_ramp_to_zero
 
     def start_ramp_to_zero(self, state):