Neutral to Ground Swells in Isolate Wet Environment

[sippy69]

Hey all, I'm new here and I have a questions.
In surgical environments (wet locations) hospitals typically use a LIMS (Line Isolation Monitor System) to have operating rooms on isolated grounding system.
I've been monitoring a facility w/ suspected power quality issues, and I have recorded many N-G swells near the 100V range.
Typically on LIMS systems, measuring from H-G & N-G will read 60V each (I know this is how the LIMS system is typically wired.

My question is this, are these swells anything to worry about?
Typically I know the I*R drop will typically read up to 2V from neutral to ground when the system is wired differently, but with an isolated grounding system such as the LIMS, I do not know if the swells I am picking up could be normal or not.
There were also transient levels recorded up to 1,400V and judging from the phase angle, they are mostly motor loads or some other non-linear load.

Thanks in advance!

 

[roger]

On the secondary side of the Isolation System Transformer you will not have a grounded conductor, so if you are monitoring the secondary of the system and seeing the swells it probably has to do with the LIM monitor checking itself periodically. SqD LIM's will self test and fault to ground as part of the periodic checks. If you can establish a repetitive time frame to the problem it may be due to something like this.

If it's on the primary side of the Isolation System Transformer it is most likely something upstream of the feeder to the Transformer and not the system itself.

Roger

 

[sippy69]

Thanks Rog,

Typically I can only measure the secondary side.
Here are some readings from the Fluke VR101 Event Recorder.

Would the LIMS ground monitor put out this kind of amplitude though?

1198 11/13/2006 14:11 Outage 0 Vrms 11/15/2006 12:45
1197 11/13/2006 8:59 Outage 0 Vrms 4:16:32
1196 11/10/2006 15:54 Outage 0 Vrms 11/13/2006 6:06
1195 11/10/2006 15:45 N-G Swell 64 Vrms 0:08:04
1194 11/09/2006 15:28 Outage 0 Vrms 11/10/2006 15:46
1193 11/09/2006 15:19 N-G Swell 60 Vrms 0:09:20
1192 11/09/2006 15:19 Outage 0 Vrms 18.0 cycles
1191 11/09/2006 15:19 1 N-G Transient -740 Vp 52?
1190 11/09/2006 15:19 1 H-N Transient -750 Vp 52?
1189 11/09/2006 15:19 N-G Swell 59 Vrms 15.0 cycles
1188 11/09/2006 14:08 Outage 0 Vrms 1:11:20
1187 11/09/2006 13:23 N-G Swell 59 Vrms 0:44:56
1186 11/09/2006 13:23 N-G Swell 58 Vrms 24.6 seconds
1185 11/09/2006 11:49 Outage 0 Vrms 1:34:00
1184 11/09/2006 10:38 N-G Swell 58 Vrms 1:10:56
1183 11/08/2006 14:45 Outage 0 Vrms 19:52:56
1182 11/08/2006 14:45 N-G Swell 55 Vrms 21.0 cycles
1181 11/08/2006 14:45 Outage 0 Vrms 26.5 cycles
1180 11/08/2006 14:45 1 N-G Transient -770 Vp 10?
1179 11/08/2006 14:45 1 H-N Transient -740 Vp 10?
1178 11/08/2006 14:45 N-G Swell 54 Vrms 20.0 cycles
1177 11/08/2006 14:45 Outage 0 Vrms 1.1 seconds
1176 11/08/2006 14:45 1 N-G Transient -930 Vp 301?
1175 11/08/2006 14:45 1 H-N Transient -880 Vp 301?
1174 11/08/2006 14:45 N-G Swell 53 Vrms 4.0 cycles
1173 11/08/2006 14:45 Outage 0 Vrms 24.0 cycles
1172 11/08/2006 14:44 N-G Swell 55 Vrms 88.8 seconds
1171 11/08/2006 14:44 N-G Swell 53 Vrms 2.0 seconds
1170 11/08/2006 14:44 N-G Swell 53 Vrms 4.0 seconds
1169 11/08/2006 14:43 N-G Swell 53 Vrms 2.6 seconds
1168 11/08/2006 14:43 N-G Swell 53 Vrms 22.2 seconds
1167 11/08/2006 14:40 N-G Swell 54 Vrms 0:02:38
1166 11/08/2006 14:40 N-G Swell 53 Vrms 1.9 seconds
1165 11/08/2006 14:40 N-G Swell 53 Vrms 26.0 cycles
1164 11/08/2006 14:39 N-G Swell 54 Vrms 61.5 seconds
1163 11/08/2006 14:39 N-G Swell 51 Vrms 2.0 cycles
1162 11/08/2006 14:39 N-G Swell 51 Vrms 2.0 cycles
1161 11/08/2006 14:39 N-G Swell 51 Vrms 2.0 cycles
1160 11/08/2006 14:39 N-G Swell 51 Vrms 2.0 cycles
1159 11/08/2006 14:39 N-G Swell 54 Vrms 10.2 seconds
1158 11/08/2006 14:39 N-G Swell 52 Vrms 7.0 cycles
1157 11/08/2006 14:39 N-G Swell 52 Vrms 2.0 cycles
1156 11/08/2006 14:39 N-G Swell 52 Vrms 3.0 cycles
1155 11/08/2006 14:39 N-G Swell 52 Vrms 3.0 cycles
1154 11/08/2006 14:39 N-G Swell 51 Vrms 2.0 cycles
1153 11/08/2006 14:39 N-G Swell 51 Vrms 2.0 cycles
1152 11/08/2006 14:39 N-G Swell 51 Vrms 2.0 cycles
1151 11/08/2006 14:38 N-G Swell 54 Vrms 65.3 seconds
1150 11/08/2006 14:38 N-G Swell 53 Vrms 3.5 seconds
1149 11/08/2006 14:27 N-G Swell 54 Vrms 0:11:28
1148 11/08/2006 14:27 N-G Swell 53 Vrms 35.0 cycles

 

[roger]

The readings look fine for the secondary L1 side of the transformer. What the recorder is showing as neutral to ground is actually the "Ungrounded" L1 conductor reading to ground, which would be roughly half the total L1-L2 voltage.

In this system there is no neutral, it is a leg to leg (or phase to phase) voltage. 60v + 60 v = 120v

Roger

 

[sippy69]

Sounds good. Thanks again Rog!

 

[roger]

You're welcome.

Roger